The UltraViolet imaging of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey Fields
(UVCANDELS) program provides Hubble Space Telescope (HST)/UVIS F275W imaging for four CANDELS
fields. We combine this UV imaging with existing HST/near-IR grism spectroscopy from 3D-HST+AGHAST to
directly compare the resolved rest-frame UV and Hα emission for a sample of 979 galaxies at 0.7 < z < 1.5,
spanning a range in stellar mass of 108−11.5 Me. Using a stacking analysis, we perform a resolved comparison
between homogenized maps of rest-UV and Hα to compute the average UV-to-Hα luminosity ratio (an indicator of
burstiness in star formation) as a function of galactocentric radius. We find that galaxies below stellar mass of
∼109.5 Me, at all radii, have a UV-to-Hα ratio higher than the equilibrium value expected from constant star
formation, indicating a significant contribution from bursty star formation. Even for galaxies with stellar mass
109.5 Me, the UV-to-Hα ratio is elevated toward their outskirts (R/Reff > 1.5), suggesting that bursty star
formation is likely prevalent in the outskirts of even the most massive galaxies, but is likely overshadowed by their
brighter cores. Furthermore, we present the UV-to-Hα ratio as a function of galaxy surface brightness, a proxy for
stellar mass surface density, and find that regions below ∼107.5 Me kpc−2 are consistent with bursty star formation,
regardless of their galaxy stellar mass, potentially suggesting that local star formation is independent of global
galaxy properties at the smallest scales. Last, we find galaxies at z > 1.1 to have bursty star formation, regardless of
radius or surface brightness.
A higher efficiency_of_converting_gas_to_stars_push_galaxies_at_z_1_6_well_ab...Sérgio Sacani
Galáxias formando estrelas em taxas extremas a nove bilhões de anos atrás eram mais eficientes do que a média das galáxias atuais, descobriram os pesquisadores.
A maioria das estrelas acredita-se localizam-se na sequência principal onde quanto maior a massa da galáxia, mais eficiente ela é na formação de novas estrelas. Contudo, de vez em quando uma galáxia apresentará uma explosão de novas estrelas que brilham mais do que o resto. Uma colisão entre duas grandes galáxias é normalmente a causa dessas fases de explosões de formação de estrelas, onde o gás frio que reside nas grandes nuvens moleculares torna-se o combustível para sustentar essas altas taxas de formação de estrelas.
A questão que os astrônomos têm feito é se essas explosões de estrelas no início o universo foram o resultado de se ter um suprimento de gás abundante, ou se as galáxias convertiam o gás de maneira mais eficiente.
Um novo estudo, publicado no Astrophysical Journal Letters de 15 de Outubro, liderado por John Silverman, do Kavli Institute for Physics and Mathematics of the Universe, estudou o conteúdo do gás monóxido de carbono (CO) em sete galáxias de explosão de estrelas muito distantes, quando o universo tinha apenas 4 bilhões de anos de vida. Isso foi possível devido a capacidade do Atacama Large Millimiter/Submillimiter Array (ALMA), localizado no platô no topo da montanha no Chile, que trabalha para detectar as ondas eletromagnéticas no comprimento de onda milimétrico (importante para se estudar o gás molecular) e um nível de sensibilidade que só agora começa a ser explorado pelos astrônomos.
Os pesquisadores descobriram que a quantidade de gás CO emitido já tinha diminuído, mesmo apesar da galáxia continuar a formar estrelas em altas taxas. Essas observações são similares àquelas registradas para as galáxias de explosões de estrelas próximas da Terra atualmente, mas a quantidade da depleção de gás não foi tão rápida quanto se esperava. Isso levou os pesquisadores a concluírem que poderia haver um contínuo aumento na eficiência, dependendo em de quanto acima da taxa de se formar estrelas ela está da sequência principal.
The most luminous_galaxies_discovered_by_wiseSérgio Sacani
Artigo descreve estudo feito por astrônomos e com a ajuda da sonda WISE da NASA para identificar as galáxias do tipo ELIRGs, entre elas a mais luminosa galáxia do universo, com um buraco negro gigantesco em seu interior e localizada a cerca de 12.8 bilhões de anos de distância da Terra.
The high mass_stelar_initial_mass_function_in_m31_clustersSérgio Sacani
Em uma pesquisa feita com o Telescópio Espacial Hubble da NASA, analisando imagens de 2753 jovens, aglomerados estelares azuis, na vizinha galáxia de Andrômeda, a M31, os astrônomos descobriram que a M31 e a nossa própria galáxia, possuem uma porcentagem similar de estrelas recém-nascidas, com base na massa estudada.
Identificando qual porcentagem de estrelas tem uma massa particular, dentro de um aglomerado, ou sua Função de Massa Inicial, IMF, os cientistas podem interpretar melhor a luz de galáxias distantes e entender a história de formação das estrelas no universo.
A intensa pesquisa, agrupou 414 mosaicos fotográficos feitos pelo Hubble da M31, uma colaboração única feita entre astrônomos, cientistas cidadãos, voluntários que forneceram uma ajuda valiosa em analisar a montanha de dados do Hubble.
“Dada a quantidade de imagens do Hubble, nosso estudo da IMF, não seria possível sem a ajuda dos cientistas cidadãos”, disse Daniel Weisz, da Universidade de Washington em Seatle. Weisz, é o principal autor do artigo que foi publicado no dia 20 de Junho no The Astrophysical Journal.
Evidence for the_thermal_sunyaev-zeldovich_effect_associated_with_quasar_feed...Sérgio Sacani
Using a radio-quiet subsample of the Sloan Digital Sky Survey spectroscopic quasar
catalogue, spanning redshifts 0.5–3.5, we derive the mean millimetre and far-infrared
quasar spectral energy distributions (SEDs) via a stacking analysis of Atacama Cosmology
Telescope and Herschel-Spectral and Photometric Imaging REceiver data. We
constrain the form of the far-infrared emission and find 3σ–4σ evidence for the thermal
Sunyaev-Zel’dovich (SZ) effect, characteristic of a hot ionized gas component with
thermal energy (6.2 ± 1.7) × 1060 erg. This amount of thermal energy is greater than
expected assuming only hot gas in virial equilibrium with the dark matter haloes of
(1 − 5) × 1012h
−1M that these systems are expected to occupy, though the highest
quasar mass estimates found in the literature could explain a large fraction of this
energy. Our measurements are consistent with quasars depositing up to (14.5±3.3) τ
−1
8
per cent of their radiative energy into their circumgalactic environment if their typical
period of quasar activity is τ8 × 108 yr. For high quasar host masses, ∼ 1013h
−1M,
this percentage will be reduced. Furthermore, the uncertainty on this percentage is
only statistical and additional systematic uncertainties enter at the 40 per cent level.
The SEDs are dust dominated in all bands and we consider various models for dust
emission. While sufficiently complex dust models can obviate the SZ effect, the SZ
interpretation remains favoured at the 3σ–4σ level for most models.
Todo mundo sabe que os raios produzidos pela Estrela da Morte em Guerra nas Estrelas não pode existir na vida real, porém no universo existem fenômenos que as vezes conseguem superar até a mais surpreendente ficção.
A galáxia Pictor A, é um desses objetos que possuem fenômenos tão espetaculares quanto aqueles exibidos no cinema. Essa galáxia localiza-se a cerca de 500 milhões de anos-luz da Terra e possui um buraco negro supermassivo no seu centro. Uma grande quantidade de energia gravitacional é lançada, à medida que o material cai em direção ao horizonte de eventos, o ponto sem volta ao redor do buraco negro. Essa energia produz um enorme jato de partículas que viajam a uma velocidade próxima da velocidade da luz no espaço intergaláctico, chamado de jato relativístico.
Para obter imagens desse jato, os cientistas usaram o Observatório de Raios-X Chandra, da NASA várias vezes durante 15 anos. Os dados do Chandra, apresentados em azul nas imagens, foram combinados com os dados obtidos em ondas de rádio a partir do Australia Telescope Compact Array, e são aparesentados em vermelho nas imagens.
A higher efficiency_of_converting_gas_to_stars_push_galaxies_at_z_1_6_well_ab...Sérgio Sacani
Galáxias formando estrelas em taxas extremas a nove bilhões de anos atrás eram mais eficientes do que a média das galáxias atuais, descobriram os pesquisadores.
A maioria das estrelas acredita-se localizam-se na sequência principal onde quanto maior a massa da galáxia, mais eficiente ela é na formação de novas estrelas. Contudo, de vez em quando uma galáxia apresentará uma explosão de novas estrelas que brilham mais do que o resto. Uma colisão entre duas grandes galáxias é normalmente a causa dessas fases de explosões de formação de estrelas, onde o gás frio que reside nas grandes nuvens moleculares torna-se o combustível para sustentar essas altas taxas de formação de estrelas.
A questão que os astrônomos têm feito é se essas explosões de estrelas no início o universo foram o resultado de se ter um suprimento de gás abundante, ou se as galáxias convertiam o gás de maneira mais eficiente.
Um novo estudo, publicado no Astrophysical Journal Letters de 15 de Outubro, liderado por John Silverman, do Kavli Institute for Physics and Mathematics of the Universe, estudou o conteúdo do gás monóxido de carbono (CO) em sete galáxias de explosão de estrelas muito distantes, quando o universo tinha apenas 4 bilhões de anos de vida. Isso foi possível devido a capacidade do Atacama Large Millimiter/Submillimiter Array (ALMA), localizado no platô no topo da montanha no Chile, que trabalha para detectar as ondas eletromagnéticas no comprimento de onda milimétrico (importante para se estudar o gás molecular) e um nível de sensibilidade que só agora começa a ser explorado pelos astrônomos.
Os pesquisadores descobriram que a quantidade de gás CO emitido já tinha diminuído, mesmo apesar da galáxia continuar a formar estrelas em altas taxas. Essas observações são similares àquelas registradas para as galáxias de explosões de estrelas próximas da Terra atualmente, mas a quantidade da depleção de gás não foi tão rápida quanto se esperava. Isso levou os pesquisadores a concluírem que poderia haver um contínuo aumento na eficiência, dependendo em de quanto acima da taxa de se formar estrelas ela está da sequência principal.
The most luminous_galaxies_discovered_by_wiseSérgio Sacani
Artigo descreve estudo feito por astrônomos e com a ajuda da sonda WISE da NASA para identificar as galáxias do tipo ELIRGs, entre elas a mais luminosa galáxia do universo, com um buraco negro gigantesco em seu interior e localizada a cerca de 12.8 bilhões de anos de distância da Terra.
The high mass_stelar_initial_mass_function_in_m31_clustersSérgio Sacani
Em uma pesquisa feita com o Telescópio Espacial Hubble da NASA, analisando imagens de 2753 jovens, aglomerados estelares azuis, na vizinha galáxia de Andrômeda, a M31, os astrônomos descobriram que a M31 e a nossa própria galáxia, possuem uma porcentagem similar de estrelas recém-nascidas, com base na massa estudada.
Identificando qual porcentagem de estrelas tem uma massa particular, dentro de um aglomerado, ou sua Função de Massa Inicial, IMF, os cientistas podem interpretar melhor a luz de galáxias distantes e entender a história de formação das estrelas no universo.
A intensa pesquisa, agrupou 414 mosaicos fotográficos feitos pelo Hubble da M31, uma colaboração única feita entre astrônomos, cientistas cidadãos, voluntários que forneceram uma ajuda valiosa em analisar a montanha de dados do Hubble.
“Dada a quantidade de imagens do Hubble, nosso estudo da IMF, não seria possível sem a ajuda dos cientistas cidadãos”, disse Daniel Weisz, da Universidade de Washington em Seatle. Weisz, é o principal autor do artigo que foi publicado no dia 20 de Junho no The Astrophysical Journal.
Evidence for the_thermal_sunyaev-zeldovich_effect_associated_with_quasar_feed...Sérgio Sacani
Using a radio-quiet subsample of the Sloan Digital Sky Survey spectroscopic quasar
catalogue, spanning redshifts 0.5–3.5, we derive the mean millimetre and far-infrared
quasar spectral energy distributions (SEDs) via a stacking analysis of Atacama Cosmology
Telescope and Herschel-Spectral and Photometric Imaging REceiver data. We
constrain the form of the far-infrared emission and find 3σ–4σ evidence for the thermal
Sunyaev-Zel’dovich (SZ) effect, characteristic of a hot ionized gas component with
thermal energy (6.2 ± 1.7) × 1060 erg. This amount of thermal energy is greater than
expected assuming only hot gas in virial equilibrium with the dark matter haloes of
(1 − 5) × 1012h
−1M that these systems are expected to occupy, though the highest
quasar mass estimates found in the literature could explain a large fraction of this
energy. Our measurements are consistent with quasars depositing up to (14.5±3.3) τ
−1
8
per cent of their radiative energy into their circumgalactic environment if their typical
period of quasar activity is τ8 × 108 yr. For high quasar host masses, ∼ 1013h
−1M,
this percentage will be reduced. Furthermore, the uncertainty on this percentage is
only statistical and additional systematic uncertainties enter at the 40 per cent level.
The SEDs are dust dominated in all bands and we consider various models for dust
emission. While sufficiently complex dust models can obviate the SZ effect, the SZ
interpretation remains favoured at the 3σ–4σ level for most models.
Todo mundo sabe que os raios produzidos pela Estrela da Morte em Guerra nas Estrelas não pode existir na vida real, porém no universo existem fenômenos que as vezes conseguem superar até a mais surpreendente ficção.
A galáxia Pictor A, é um desses objetos que possuem fenômenos tão espetaculares quanto aqueles exibidos no cinema. Essa galáxia localiza-se a cerca de 500 milhões de anos-luz da Terra e possui um buraco negro supermassivo no seu centro. Uma grande quantidade de energia gravitacional é lançada, à medida que o material cai em direção ao horizonte de eventos, o ponto sem volta ao redor do buraco negro. Essa energia produz um enorme jato de partículas que viajam a uma velocidade próxima da velocidade da luz no espaço intergaláctico, chamado de jato relativístico.
Para obter imagens desse jato, os cientistas usaram o Observatório de Raios-X Chandra, da NASA várias vezes durante 15 anos. Os dados do Chandra, apresentados em azul nas imagens, foram combinados com os dados obtidos em ondas de rádio a partir do Australia Telescope Compact Array, e são aparesentados em vermelho nas imagens.
XUE: Molecular Inventory in the Inner Region of an Extremely Irradiated Proto...Sérgio Sacani
We present the first results of the eXtreme UV Environments (XUE) James Webb Space Telescope (JWST)
program, which focuses on the characterization of planet-forming disks in massive star-forming regions. These
regions are likely representative of the environment in which most planetary systems formed. Understanding the
impact of environment on planet formation is critical in order to gain insights into the diversity of the observed
exoplanet populations. XUE targets 15 disks in three areas of NGC 6357, which hosts numerous massive OB stars,
including some of the most massive stars in our Galaxy. Thanks to JWST, we can, for the first time, study the effect
of external irradiation on the inner (<10 au), terrestrial-planet-forming regions of protoplanetary disks. In this study,
we report on the detection of abundant water, CO, 12CO2, HCN, and C2H2 in the inner few au of XUE 1, a highly
irradiated disk in NGC 6357. In addition, small, partially crystalline silicate dust is present at the disk surface. The
derived column densities, the oxygen-dominated gas-phase chemistry, and the presence of silicate dust are
surprisingly similar to those found in inner disks located in nearby, relatively isolated low-mass star-forming
regions. Our findings imply that the inner regions of highly irradiated disks can retain similar physical and chemical
conditions to disks in low-mass star-forming regions, thus broadening the range of environments with similar
conditions for inner disk rocky planet formation to the most extreme star-forming regions in our Galaxy.
Spirals and clumps in V960 Mon: signs of planet formation via gravitational i...Sérgio Sacani
The formation of giant planets has traditionally been divided into two pathways: core accretion and gravitational instability. However, in recent years, gravitational instability has become less favored, primarily due
to the scarcity of observations of fragmented protoplanetary disks around young stars and low occurrence rate
of massive planets on very wide orbits. In this study, we present a SPHERE/IRDIS polarized light observation
of the young outbursting object V960 Mon. The image reveals a vast structure of intricately shaped scattered
light with several spiral arms. This finding motivated a re-analysis of archival ALMA 1.3 mm data acquired
just two years after the onset of the outburst of V960 Mon. In these data, we discover several clumps of continuum emission aligned along a spiral arm that coincides with the scattered light structure. We interpret the
localized emission as fragments formed from a spiral arm under gravitational collapse. Estimating the mass of
solids within these clumps to be of several Earth masses, we suggest this observation to be the first evidence of
gravitational instability occurring on planetary scales. This study discusses the significance of this finding for
planet formation and its potential connection with the outbursting state of V960 Mon.
Exomoons & Exorings with the Habitable Worlds Observatory I: On the Detection...Sérgio Sacani
The highest priority recommendation of the Astro2020 Decadal Survey for space-based astronomy
was the construction of an observatory capable of characterizing habitable worlds. In this paper series
we explore the detectability of and interference from exomoons and exorings serendipitously observed
with the proposed Habitable Worlds Observatory (HWO) as it seeks to characterize exoplanets, starting
in this manuscript with Earth-Moon analog mutual events. Unlike transits, which only occur in systems
viewed near edge-on, shadow (i.e., solar eclipse) and lunar eclipse mutual events occur in almost every
star-planet-moon system. The cadence of these events can vary widely from ∼yearly to multiple events
per day, as was the case in our younger Earth-Moon system. Leveraging previous space-based (EPOXI)
lightcurves of a Moon transit and performance predictions from the LUVOIR-B concept, we derive
the detectability of Moon analogs with HWO. We determine that Earth-Moon analogs are detectable
with observation of ∼2-20 mutual events for systems within 10 pc, and larger moons should remain
detectable out to 20 pc. We explore the extent to which exomoon mutual events can mimic planet
features and weather. We find that HWO wavelength coverage in the near-IR, specifically in the 1.4 µm
water band where large moons can outshine their host planet, will aid in differentiating exomoon signals
from exoplanet variability. Finally, we predict that exomoons formed through collision processes akin
to our Moon are more likely to be detected in younger systems, where shorter orbital periods and
favorable geometry enhance the probability and frequency of mutual events.
EXTINCTION AND THE DIMMING OF KIC 8462852Sérgio Sacani
To test alternative hypotheses for the behavior of KIC 8462852, we obtained measurements of the star
over a wide wavelength range from the UV to the mid-infrared from October 2015 through December
2016, using Swift, Spitzer and at AstroLAB IRIS. The star faded in a manner similar to the longterm
fading seen in Kepler data about 1400 days previously. The dimming rate for the entire period
reported is 22.1 ± 9.7 milli-mag yr−1
in the Swift wavebands, with amounts of 21.0 ± 4.5 mmag in
the groundbased B measurements, 14.0 ± 4.5 mmag in V , and 13.0 ± 4.5 in R, and a rate of 5.0 ± 1.2
mmag yr−1 averaged over the two warm Spitzer bands. Although the dimming is small, it is seen at
& 3 σ by three different observatories operating from the UV to the IR. The presence of long-term
secular dimming means that previous SED models of the star based on photometric measurements
taken years apart may not be accurate. We find that stellar models with Tef f = 7000 - 7100 K and
AV ∼ 0.73 best fit the Swift data from UV to optical. These models also show no excess in the
near-simultaneous Spitzer photometry at 3.6 and 4.5 µm, although a longer wavelength excess from
a substantial debris disk is still possible (e.g., as around Fomalhaut). The wavelength dependence of
the fading favors a relatively neutral color (i.e., RV & 5, but not flat across all the bands) compared
with the extinction law for the general ISM (RV = 3.1), suggesting that the dimming arises from
circumstellar material
UV and Hα HST observations of 6 GASP jellyfish galaxiesSérgio Sacani
Star-forming, Hα-emitting clumps are found embedded in the gaseous tails of galaxies undergoing
intense ram pressure stripping in galaxy clusters, so-called jellyfish galaxies. These clumps offer a
unique opportunity to study star formation under extreme conditions, in the absence of an underlying
disk and embedded within the hot intracluster medium. Yet, a comprehensive, high spatial resolution
study of these systems is missing. We obtained UVIS/HST data to observe the first statistical sample
of clumps in the tails and disks of six jellyfish galaxies from the GASP survey; we used a combination
of broad-band (UV to I) filters and a narrow-band Hα filter. HST observations are needed to study
the sizes, stellar masses and ages of the clumps and their clustering hierarchy. These observations will
be used to study the clump scaling relations, the universality of the star formation process and verify
whether a disk is irrelevant, as hinted by jellyfish galaxy results. This paper presents the observations,
data reduction strategy, and some general results based on the preliminary data analysis. The UVIS
high spatial resolution gives an unprecedented sharp view of the complex structure of the inner regions
of the galaxies and of the substructures in the galaxy disks. We found clear signatures of stripping
in regions very close in projection to the galactic disk. The star-forming regions in the stripped tails
are extremely bright and compact while we did not detect a significant number of star-forming clumps
outside those detected by MUSE. The paper finally presents the development plan for the project.
UV and Hα HST observations of 6 GASP jellyfish galaxiesSérgio Sacani
Star-forming, Hα-emitting clumps are found embedded in the gaseous tails of galaxies undergoing
intense ram pressure stripping in galaxy clusters, so-called jellyfish galaxies. These clumps offer a
unique opportunity to study star formation under extreme conditions, in the absence of an underlying
disk and embedded within the hot intracluster medium. Yet, a comprehensive, high spatial resolution
study of these systems is missing. We obtained UVIS/HST data to observe the first statistical sample
of clumps in the tails and disks of six jellyfish galaxies from the GASP survey; we used a combination
of broad-band (UV to I) filters and a narrow-band Hα filter. HST observations are needed to study
the sizes, stellar masses and ages of the clumps and their clustering hierarchy. These observations will
be used to study the clump scaling relations, the universality of the star formation process and verify
whether a disk is irrelevant, as hinted by jellyfish galaxy results. This paper presents the observations,
data reduction strategy, and some general results based on the preliminary data analysis. The UVIS
high spatial resolution gives an unprecedented sharp view of the complex structure of the inner regions
of the galaxies and of the substructures in the galaxy disks. We found clear signatures of stripping
in regions very close in projection to the galactic disk. The star-forming regions in the stripped tails
are extremely bright and compact while we did not detect a significant number of star-forming clumps
outside those detected by MUSE. The paper finally presents the development plan for the project.
UV and Hα HST observations of 6 GASP jellyfish galaxiesSérgio Sacani
Star-forming, Hα-emitting clumps are found embedded in the gaseous tails of galaxies undergoing
intense ram pressure stripping in galaxy clusters, so-called jellyfish galaxies. These clumps offer a
unique opportunity to study star formation under extreme conditions, in the absence of an underlying
disk and embedded within the hot intracluster medium. Yet, a comprehensive, high spatial resolution
study of these systems is missing. We obtained UVIS/HST data to observe the first statistical sample
of clumps in the tails and disks of six jellyfish galaxies from the GASP survey; we used a combination
of broad-band (UV to I) filters and a narrow-band Hα filter. HST observations are needed to study
the sizes, stellar masses and ages of the clumps and their clustering hierarchy. These observations will
be used to study the clump scaling relations, the universality of the star formation process and verify
whether a disk is irrelevant, as hinted by jellyfish galaxy results. This paper presents the observations,
data reduction strategy, and some general results based on the preliminary data analysis. The UVIS
high spatial resolution gives an unprecedented sharp view of the complex structure of the inner regions
of the galaxies and of the substructures in the galaxy disks. We found clear signatures of stripping
in regions very close in projection to the galactic disk. The star-forming regions in the stripped tails
are extremely bright and compact while we did not detect a significant number of star-forming clumps
outside those detected by MUSE. The paper finally presents the development plan for the project.
The ALMA Survey of Star Formation and Evolution in Massive Protoclusters with...Sérgio Sacani
The ALMA Survey of Star Formation and Evolution in Massive Protoclusters with Blue Profiles
(ASSEMBLE) aims to investigate the process of mass assembly and its connection to high-mass star
formation theories in protoclusters in a dynamic view. We observed 11 massive (Mclump ≳ 103 M⊙),
luminous (Lbol ≳ 104 L⊙), and blue-profile (infall signature) clumps by ALMA with resolution of
∼2200–5500 au (median value of 3500 au) at 350 GHz (870 µm). 248 dense cores were identified, including 106 cores showing protostellar signatures and 142 prestellar core candidates. Compared to
early-stage infrared dark clouds (IRDCs) by ASHES, the core mass and surface density within the
ASSEMBLE clumps exhibited significant increment, suggesting concurrent core accretion during the
evolution of the clumps. The maximum mass of prestellar cores was found to be 2 times larger than
that in IRDCs, indicating that evolved protoclusters have the potential to harbor massive prestellar
cores. The mass relation between clumps and their most massive core (MMCs) is observed in ASSEMBLE but not in IRDCs, which is suggested to be regulated by multiscale mass accretion. The
mass correlation between the core clusters and their MMCs has a steeper slope compared to that
observed in stellar clusters, which can be due to fragmentation of the MMC and stellar multiplicity.
We observe a decrease in core separation and an increase in central concentration as protoclusters
evolve. We confirm primordial mass segregation in the ASSEMBLE protoclusters, possibly resulting
from gravitational concentration and/or gas accretion.
The Lyα Reference Sample. XIV. Lyα Imaging of 45 Low-redshift Star-forming Ga...Sérgio Sacani
We present Lyα imaging of 45 low-redshift star-forming galaxies observed with the Hubble Space Telescope. The
galaxies have been selected to have moderate to high star formation rates (SFRs) using far-ultraviolet (FUV)
luminosity and Hα equivalent width criteria, but no constraints on Lyα luminosity. We employ a pixel stellar
continuum fitting code to obtain accurate continuum-subtracted Lyα, Hα, and Hβ maps. We find that Lyα is less
concentrated than FUV and optical line emission in almost all galaxies with significant Lyα emission. We present
global measurements of Lyα and other quantities measured in apertures designed to capture all of the Lyα
emission. We then show how the escape fraction of Lyα relates to a number of other measured quantities (mass,
metallicity, star formation, ionization parameter, and extinction). We find that the escape fraction is strongly
anticorrelated with nebular and stellar extinction, weakly anticorrelated with stellar mass, but no conclusive
evidence for correlations with other quantities. We show that Lyα escape fractions are inconsistent with common
dust extinction laws, and discuss how a combination of radiative transfer effects and clumpy dust models can help
resolve the discrepancies. We present an SFR calibration based on Lyα luminosity, where the equivalent width of
Lyα is used to correct for nonunity escape fraction, and show that this relation provides a reasonably accurate SFR
estimate. We also show stacked growth curves of Lyα for the galaxies that can be used to find aperture loss
fractions at a given physical radius.
Merging galaxy clusters leave long-lasting signatures on the baryonic and non-baryonic cluster constituents,
including shock fronts, cold fronts, X-ray substructure, radio halos, and offsets between the dark matter (DM) and
the gas components. Using observations from Chandra, the Jansky Very Large Array, the Giant Metrewave Radio
Telescope, and the Hubble Space Telescope, we present a multiwavelength analysis of the merging Frontier Fields
cluster MACS J0416.1-2403 (z = 0.396), which consists of NE and SW subclusters whose cores are separated on
the sky by ∼250 kpc. We find that the NE subcluster has a compact core and hosts an X-ray cavity, yet it is not a
cool core. Approximately 450 kpc south–southwest of the SW subcluster, we detect a density discontinuity that
corresponds to a compression factor of ∼1.5. The discontinuity was most likely caused by the interaction of the
SW subcluster with a less massive structure detected in the lensing maps SW of the subclusterʼs center. For both
the NE and the SW subclusters, the DM and the gas components are well-aligned, suggesting that MACS J0416.1-
2403 is a pre-merging system. The cluster also hosts a radio halo, which is unusual for a pre-merging system. The
halo has a 1.4 GHz power of (1.3 ± 0.3) × 1024WHz−1, which is somewhat lower than expected based on the
X-ray luminosity of the cluster if the spectrum of the halo is not ultra-steep. We suggest that we are either
witnessing the birth of a radio halo, or have discovered a rare ultra-steep spectrum halo.
The Variable Detection of Atmospheric Escape around the Young, Hot Neptune AU...Sérgio Sacani
Photoevaporation is a potential explanation for several features within exoplanet demographics. Atmospheric escape
observed in young Neptune-sized exoplanets can provide insight into and characterize which mechanisms drive this
evolution and at what times they dominate. AU Mic b is one such exoplanet, slightly larger than Neptune (4.19 R⊕).
It closely orbits a 23 Myr pre-main-sequence M dwarf with an orbital period of 8.46 days. We obtained two visits of
AU Mic b at Lyα with Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph. One flare within the
first HST visit is characterized and removed from our search for a planetary transit. We present a nondetection in our
first visit, followed by the detection of escaping neutral hydrogen ahead of the planet in our second visit. The outflow
absorbed ∼30% of the star’s Lyα blue wing 2.5 hr before the planet’s white-light transit. We estimate that the
highest-velocity escaping material has a column density of 1013.96 cm−2 and is moving 61.26 km s−1 away from the
host star. AU Mic b’s large high-energy irradiation could photoionize its escaping neutral hydrogen in 44 minutes,
rendering it temporarily unobservable. Our time-variable Lyα transit ahead of AU Mic b could also be explained by
an intermediate stellar wind strength from AU Mic that shapes the escaping material into a leading tail. Future Lyα
observations of this system will confirm and characterize the unique variable nature of its Lyα transit, which,
combined with modeling, will tune the importance of stellar wind and photoionization.
Is the atmosphere of the ultra-hot Jupiter WASP-121 b variable?Sérgio Sacani
We present a comprehensive analysis of the Hubble Space Telescope observations of the atmosphere
of WASP-121 b, a ultra-hot Jupiter. After reducing the transit, eclipse, and phase-curve observations
with a uniform methodology and addressing the biases from instrument systematics, sophisticated
atmospheric retrievals are used to extract robust constraints on the thermal structure, chemistry, and
cloud properties of the atmosphere. Our analysis shows that the observations are consistent with a
strong thermal inversion beginning at ∼104 Pa on the dayside, solar to subsolar metallicity Z (i.e.,
−0.77 < log(Z) < 0.05), and super-solar C/O ratio (i.e., 0.59 < C/O < 0.87). More importantly,
utilizing the high signal-to-noise ratio and repeated observations of the planet, we identify the following
unambiguous time-varying signals in the data: i) a shift of the putative hotspot offset between the
two phase-curves and ii) varying spectral signatures in the transits and eclipses. By simulating the
global dynamics of WASP-121 b atmosphere at high-resolution, we show that the identified signals are
consistent with quasi-periodic weather patterns, hence atmospheric variability, with signatures at the
level probed by the observations (∼5% to ∼10%) that change on a timescale of ∼5 planet days; in
the simulations, the weather patterns arise from the formation and movement of storms and fronts,
causing hot (as well as cold) patches of atmosphere to deform, separate, and mix in time.
Search for an Isotropic Gravitational-wave Background with the Parkes Pulsar ...Sérgio Sacani
Pulsar timing arrays aim to detect nanohertz-frequency gravitational waves (GWs). A background of GWs
modulates pulsar arrival times and manifests as a stochastic process, common to all pulsars, with a signature spatial
correlation. Here we describe a search for an isotropic stochastic gravitational-wave background (GWB) using
observations of 30 millisecond pulsars from the third data release of the Parkes Pulsar Timing Array (PPTA), which
spans 18 yr. Using current Bayesian inference techniques we recover and characterize a common-spectrum noise
process. Represented as a strain spectrum h Af c 1yr = -1 a ( ) , we measure A 3.1 10 0.9 = ´ 1.3 15 -
+ - and
α = −0.45 ± 0.20, respectively (median and 68% credible interval). For a spectral index of α = −2/3,
corresponding to an isotropic background of GWs radiated by inspiraling supermassive black hole binaries, we
recover an amplitude of A 2.04 10 0.22 = ´ 0.25 15 -
+ - . However, we demonstrate that the apparent signal strength is timedependent, as the first half of our data set can be used to place an upper limit on A that is in tension with the inferred
common-spectrum amplitude using the complete data set. We search for spatial correlations in the observations by
hierarchically analyzing individual pulsar pairs, which also allows for significance validation through randomizing
pulsar positions on the sky. For a process with α = −2/3, we measure spatial correlations consistent with a GWB,
with an estimated false-alarm probability of p 0.02 (approx. 2σ). The long timing baselines of the PPTA and the
access to southern pulsars will continue to play an important role in the International Pulsar Timing Array.
First Direct Imaging of a Kelvin–Helmholtz Instability by PSP/WISPRSérgio Sacani
We present a comprehensive analysis aimed at proving the hypothesis that a train of small-scale features observed by the Wide-field Imager (WISPR) onboard the Parker Solar Probe (PSP) are the signature of a Kelvin–Helmholtz instability (KHI). These features were seen near the flank of a Coronal Mass Ejection (CME) wake between 7.5 Re and 9.5Re, lasting for about 30 minutes. The CME was a slow event, associated with a streamer blowout. We analyzed the size of the eddies and found growth during their evolution while maintaining separation distances and alignment typical of Kelvin–Helmholtz vortexes. We then assessed the magnetic field conditions that would make the observation of such an instability plausible. Two methods were used to cross-check our findings. The measured thickness of the boundary layer supports KHI candidacy, and the estimated linear growth rate suggests nonlinear saturation within the expected timescale. We conclude that a KHI is a plausible explanation for the observed features, and therefore that such instabilities might exist in the low and middle solar corona (within ∼15 Re) and can be detected in white light observations. Their observation, however, might be rare due to stringent conditions like the observer’s proximity, suitable viewing circumstances, magnetic field topology, and flow properties. This study highlights the unique capability of PSP/WISPR in observing such phenomena, especially as PSP perihelia reach closer distances to the Sun.
Is Betelgeuse Really Rotating? Synthetic ALMA Observations of Large-scale Con...Sérgio Sacani
The evolved stages of massive stars are poorly understood, but invaluable constraints can be derived from spatially resolved observations of nearby red supergiants, such as Betelgeuse. Atacama Large Millimeter/submillimeter Array (ALMA) observations of Betelgeuse showing a dipolar velocity field have been interpreted as evidence for a projected rotation rate of about 5 km s−1. This is 2 orders of magnitude larger than predicted by single-star evolution, which led to suggestions that Betelgeuse is a binary merger. We propose instead that large-scale convective motions can mimic rotation, especially if they are only partially resolved. We support this claim with 3D CO5BOLDsimulations of nonrotating red supergiants that we postprocessed to predict ALMA images and SiO spectra. We show that our synthetic radial velocity maps have a 90% chance of being falsely interpreted as evidence for a projected rotation rate of 2 km s−1 or larger for our fiducial simulation. We conclude that we need at least another ALMA observation to firmly establish whether Betelgeuse is indeed rapidly rotating. Such observations would also provide insight into the role of angular momentum and binary interaction in the late evolutionary stages. The data will further probe the structure and complex physical processes in the atmospheres of red supergiants, which are immediate progenitors of supernovae and are believed to be essential in the formation of gravitational-wave sources.
EWOCS-I: The catalog of X-ray sources in Westerlund 1 from the Extended Weste...Sérgio Sacani
Context. With a mass exceeding several 104 M⊙ and a rich and dense population of massive stars, supermassive young star clusters
represent the most massive star-forming environment that is dominated by the feedback from massive stars and gravitational interactions
among stars.
Aims. In this paper we present the Extended Westerlund 1 and 2 Open Clusters Survey (EWOCS) project, which aims to investigate
the influence of the starburst environment on the formation of stars and planets, and on the evolution of both low and high mass stars.
The primary targets of this project are Westerlund 1 and 2, the closest supermassive star clusters to the Sun.
Methods. The project is based primarily on recent observations conducted with the Chandra and JWST observatories. Specifically,
the Chandra survey of Westerlund 1 consists of 36 new ACIS-I observations, nearly co-pointed, for a total exposure time of 1 Msec.
Additionally, we included 8 archival Chandra/ACIS-S observations. This paper presents the resulting catalog of X-ray sources within
and around Westerlund 1. Sources were detected by combining various existing methods, and photon extraction and source validation
were carried out using the ACIS-Extract software.
Results. The EWOCS X-ray catalog comprises 5963 validated sources out of the 9420 initially provided to ACIS-Extract, reaching a
photon flux threshold of approximately 2 × 10−8 photons cm−2
s
−1
. The X-ray sources exhibit a highly concentrated spatial distribution,
with 1075 sources located within the central 1 arcmin. We have successfully detected X-ray emissions from 126 out of the 166 known
massive stars of the cluster, and we have collected over 71 000 photons from the magnetar CXO J164710.20-455217.
The debris of the ‘last major merger’ is dynamically youngSérgio Sacani
The Milky Way’s (MW) inner stellar halo contains an [Fe/H]-rich component with highly eccentric orbits, often referred to as the
‘last major merger.’ Hypotheses for the origin of this component include Gaia-Sausage/Enceladus (GSE), where the progenitor
collided with the MW proto-disc 8–11 Gyr ago, and the Virgo Radial Merger (VRM), where the progenitor collided with the
MW disc within the last 3 Gyr. These two scenarios make different predictions about observable structure in local phase space,
because the morphology of debris depends on how long it has had to phase mix. The recently identified phase-space folds in Gaia
DR3 have positive caustic velocities, making them fundamentally different than the phase-mixed chevrons found in simulations
at late times. Roughly 20 per cent of the stars in the prograde local stellar halo are associated with the observed caustics. Based
on a simple phase-mixing model, the observed number of caustics are consistent with a merger that occurred 1–2 Gyr ago.
We also compare the observed phase-space distribution to FIRE-2 Latte simulations of GSE-like mergers, using a quantitative
measurement of phase mixing (2D causticality). The observed local phase-space distribution best matches the simulated data
1–2 Gyr after collision, and certainly not later than 3 Gyr. This is further evidence that the progenitor of the ‘last major merger’
did not collide with the MW proto-disc at early times, as is thought for the GSE, but instead collided with the MW disc within
the last few Gyr, consistent with the body of work surrounding the VRM.
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XUE: Molecular Inventory in the Inner Region of an Extremely Irradiated Proto...Sérgio Sacani
We present the first results of the eXtreme UV Environments (XUE) James Webb Space Telescope (JWST)
program, which focuses on the characterization of planet-forming disks in massive star-forming regions. These
regions are likely representative of the environment in which most planetary systems formed. Understanding the
impact of environment on planet formation is critical in order to gain insights into the diversity of the observed
exoplanet populations. XUE targets 15 disks in three areas of NGC 6357, which hosts numerous massive OB stars,
including some of the most massive stars in our Galaxy. Thanks to JWST, we can, for the first time, study the effect
of external irradiation on the inner (<10 au), terrestrial-planet-forming regions of protoplanetary disks. In this study,
we report on the detection of abundant water, CO, 12CO2, HCN, and C2H2 in the inner few au of XUE 1, a highly
irradiated disk in NGC 6357. In addition, small, partially crystalline silicate dust is present at the disk surface. The
derived column densities, the oxygen-dominated gas-phase chemistry, and the presence of silicate dust are
surprisingly similar to those found in inner disks located in nearby, relatively isolated low-mass star-forming
regions. Our findings imply that the inner regions of highly irradiated disks can retain similar physical and chemical
conditions to disks in low-mass star-forming regions, thus broadening the range of environments with similar
conditions for inner disk rocky planet formation to the most extreme star-forming regions in our Galaxy.
Spirals and clumps in V960 Mon: signs of planet formation via gravitational i...Sérgio Sacani
The formation of giant planets has traditionally been divided into two pathways: core accretion and gravitational instability. However, in recent years, gravitational instability has become less favored, primarily due
to the scarcity of observations of fragmented protoplanetary disks around young stars and low occurrence rate
of massive planets on very wide orbits. In this study, we present a SPHERE/IRDIS polarized light observation
of the young outbursting object V960 Mon. The image reveals a vast structure of intricately shaped scattered
light with several spiral arms. This finding motivated a re-analysis of archival ALMA 1.3 mm data acquired
just two years after the onset of the outburst of V960 Mon. In these data, we discover several clumps of continuum emission aligned along a spiral arm that coincides with the scattered light structure. We interpret the
localized emission as fragments formed from a spiral arm under gravitational collapse. Estimating the mass of
solids within these clumps to be of several Earth masses, we suggest this observation to be the first evidence of
gravitational instability occurring on planetary scales. This study discusses the significance of this finding for
planet formation and its potential connection with the outbursting state of V960 Mon.
Exomoons & Exorings with the Habitable Worlds Observatory I: On the Detection...Sérgio Sacani
The highest priority recommendation of the Astro2020 Decadal Survey for space-based astronomy
was the construction of an observatory capable of characterizing habitable worlds. In this paper series
we explore the detectability of and interference from exomoons and exorings serendipitously observed
with the proposed Habitable Worlds Observatory (HWO) as it seeks to characterize exoplanets, starting
in this manuscript with Earth-Moon analog mutual events. Unlike transits, which only occur in systems
viewed near edge-on, shadow (i.e., solar eclipse) and lunar eclipse mutual events occur in almost every
star-planet-moon system. The cadence of these events can vary widely from ∼yearly to multiple events
per day, as was the case in our younger Earth-Moon system. Leveraging previous space-based (EPOXI)
lightcurves of a Moon transit and performance predictions from the LUVOIR-B concept, we derive
the detectability of Moon analogs with HWO. We determine that Earth-Moon analogs are detectable
with observation of ∼2-20 mutual events for systems within 10 pc, and larger moons should remain
detectable out to 20 pc. We explore the extent to which exomoon mutual events can mimic planet
features and weather. We find that HWO wavelength coverage in the near-IR, specifically in the 1.4 µm
water band where large moons can outshine their host planet, will aid in differentiating exomoon signals
from exoplanet variability. Finally, we predict that exomoons formed through collision processes akin
to our Moon are more likely to be detected in younger systems, where shorter orbital periods and
favorable geometry enhance the probability and frequency of mutual events.
EXTINCTION AND THE DIMMING OF KIC 8462852Sérgio Sacani
To test alternative hypotheses for the behavior of KIC 8462852, we obtained measurements of the star
over a wide wavelength range from the UV to the mid-infrared from October 2015 through December
2016, using Swift, Spitzer and at AstroLAB IRIS. The star faded in a manner similar to the longterm
fading seen in Kepler data about 1400 days previously. The dimming rate for the entire period
reported is 22.1 ± 9.7 milli-mag yr−1
in the Swift wavebands, with amounts of 21.0 ± 4.5 mmag in
the groundbased B measurements, 14.0 ± 4.5 mmag in V , and 13.0 ± 4.5 in R, and a rate of 5.0 ± 1.2
mmag yr−1 averaged over the two warm Spitzer bands. Although the dimming is small, it is seen at
& 3 σ by three different observatories operating from the UV to the IR. The presence of long-term
secular dimming means that previous SED models of the star based on photometric measurements
taken years apart may not be accurate. We find that stellar models with Tef f = 7000 - 7100 K and
AV ∼ 0.73 best fit the Swift data from UV to optical. These models also show no excess in the
near-simultaneous Spitzer photometry at 3.6 and 4.5 µm, although a longer wavelength excess from
a substantial debris disk is still possible (e.g., as around Fomalhaut). The wavelength dependence of
the fading favors a relatively neutral color (i.e., RV & 5, but not flat across all the bands) compared
with the extinction law for the general ISM (RV = 3.1), suggesting that the dimming arises from
circumstellar material
UV and Hα HST observations of 6 GASP jellyfish galaxiesSérgio Sacani
Star-forming, Hα-emitting clumps are found embedded in the gaseous tails of galaxies undergoing
intense ram pressure stripping in galaxy clusters, so-called jellyfish galaxies. These clumps offer a
unique opportunity to study star formation under extreme conditions, in the absence of an underlying
disk and embedded within the hot intracluster medium. Yet, a comprehensive, high spatial resolution
study of these systems is missing. We obtained UVIS/HST data to observe the first statistical sample
of clumps in the tails and disks of six jellyfish galaxies from the GASP survey; we used a combination
of broad-band (UV to I) filters and a narrow-band Hα filter. HST observations are needed to study
the sizes, stellar masses and ages of the clumps and their clustering hierarchy. These observations will
be used to study the clump scaling relations, the universality of the star formation process and verify
whether a disk is irrelevant, as hinted by jellyfish galaxy results. This paper presents the observations,
data reduction strategy, and some general results based on the preliminary data analysis. The UVIS
high spatial resolution gives an unprecedented sharp view of the complex structure of the inner regions
of the galaxies and of the substructures in the galaxy disks. We found clear signatures of stripping
in regions very close in projection to the galactic disk. The star-forming regions in the stripped tails
are extremely bright and compact while we did not detect a significant number of star-forming clumps
outside those detected by MUSE. The paper finally presents the development plan for the project.
UV and Hα HST observations of 6 GASP jellyfish galaxiesSérgio Sacani
Star-forming, Hα-emitting clumps are found embedded in the gaseous tails of galaxies undergoing
intense ram pressure stripping in galaxy clusters, so-called jellyfish galaxies. These clumps offer a
unique opportunity to study star formation under extreme conditions, in the absence of an underlying
disk and embedded within the hot intracluster medium. Yet, a comprehensive, high spatial resolution
study of these systems is missing. We obtained UVIS/HST data to observe the first statistical sample
of clumps in the tails and disks of six jellyfish galaxies from the GASP survey; we used a combination
of broad-band (UV to I) filters and a narrow-band Hα filter. HST observations are needed to study
the sizes, stellar masses and ages of the clumps and their clustering hierarchy. These observations will
be used to study the clump scaling relations, the universality of the star formation process and verify
whether a disk is irrelevant, as hinted by jellyfish galaxy results. This paper presents the observations,
data reduction strategy, and some general results based on the preliminary data analysis. The UVIS
high spatial resolution gives an unprecedented sharp view of the complex structure of the inner regions
of the galaxies and of the substructures in the galaxy disks. We found clear signatures of stripping
in regions very close in projection to the galactic disk. The star-forming regions in the stripped tails
are extremely bright and compact while we did not detect a significant number of star-forming clumps
outside those detected by MUSE. The paper finally presents the development plan for the project.
UV and Hα HST observations of 6 GASP jellyfish galaxiesSérgio Sacani
Star-forming, Hα-emitting clumps are found embedded in the gaseous tails of galaxies undergoing
intense ram pressure stripping in galaxy clusters, so-called jellyfish galaxies. These clumps offer a
unique opportunity to study star formation under extreme conditions, in the absence of an underlying
disk and embedded within the hot intracluster medium. Yet, a comprehensive, high spatial resolution
study of these systems is missing. We obtained UVIS/HST data to observe the first statistical sample
of clumps in the tails and disks of six jellyfish galaxies from the GASP survey; we used a combination
of broad-band (UV to I) filters and a narrow-band Hα filter. HST observations are needed to study
the sizes, stellar masses and ages of the clumps and their clustering hierarchy. These observations will
be used to study the clump scaling relations, the universality of the star formation process and verify
whether a disk is irrelevant, as hinted by jellyfish galaxy results. This paper presents the observations,
data reduction strategy, and some general results based on the preliminary data analysis. The UVIS
high spatial resolution gives an unprecedented sharp view of the complex structure of the inner regions
of the galaxies and of the substructures in the galaxy disks. We found clear signatures of stripping
in regions very close in projection to the galactic disk. The star-forming regions in the stripped tails
are extremely bright and compact while we did not detect a significant number of star-forming clumps
outside those detected by MUSE. The paper finally presents the development plan for the project.
The ALMA Survey of Star Formation and Evolution in Massive Protoclusters with...Sérgio Sacani
The ALMA Survey of Star Formation and Evolution in Massive Protoclusters with Blue Profiles
(ASSEMBLE) aims to investigate the process of mass assembly and its connection to high-mass star
formation theories in protoclusters in a dynamic view. We observed 11 massive (Mclump ≳ 103 M⊙),
luminous (Lbol ≳ 104 L⊙), and blue-profile (infall signature) clumps by ALMA with resolution of
∼2200–5500 au (median value of 3500 au) at 350 GHz (870 µm). 248 dense cores were identified, including 106 cores showing protostellar signatures and 142 prestellar core candidates. Compared to
early-stage infrared dark clouds (IRDCs) by ASHES, the core mass and surface density within the
ASSEMBLE clumps exhibited significant increment, suggesting concurrent core accretion during the
evolution of the clumps. The maximum mass of prestellar cores was found to be 2 times larger than
that in IRDCs, indicating that evolved protoclusters have the potential to harbor massive prestellar
cores. The mass relation between clumps and their most massive core (MMCs) is observed in ASSEMBLE but not in IRDCs, which is suggested to be regulated by multiscale mass accretion. The
mass correlation between the core clusters and their MMCs has a steeper slope compared to that
observed in stellar clusters, which can be due to fragmentation of the MMC and stellar multiplicity.
We observe a decrease in core separation and an increase in central concentration as protoclusters
evolve. We confirm primordial mass segregation in the ASSEMBLE protoclusters, possibly resulting
from gravitational concentration and/or gas accretion.
The Lyα Reference Sample. XIV. Lyα Imaging of 45 Low-redshift Star-forming Ga...Sérgio Sacani
We present Lyα imaging of 45 low-redshift star-forming galaxies observed with the Hubble Space Telescope. The
galaxies have been selected to have moderate to high star formation rates (SFRs) using far-ultraviolet (FUV)
luminosity and Hα equivalent width criteria, but no constraints on Lyα luminosity. We employ a pixel stellar
continuum fitting code to obtain accurate continuum-subtracted Lyα, Hα, and Hβ maps. We find that Lyα is less
concentrated than FUV and optical line emission in almost all galaxies with significant Lyα emission. We present
global measurements of Lyα and other quantities measured in apertures designed to capture all of the Lyα
emission. We then show how the escape fraction of Lyα relates to a number of other measured quantities (mass,
metallicity, star formation, ionization parameter, and extinction). We find that the escape fraction is strongly
anticorrelated with nebular and stellar extinction, weakly anticorrelated with stellar mass, but no conclusive
evidence for correlations with other quantities. We show that Lyα escape fractions are inconsistent with common
dust extinction laws, and discuss how a combination of radiative transfer effects and clumpy dust models can help
resolve the discrepancies. We present an SFR calibration based on Lyα luminosity, where the equivalent width of
Lyα is used to correct for nonunity escape fraction, and show that this relation provides a reasonably accurate SFR
estimate. We also show stacked growth curves of Lyα for the galaxies that can be used to find aperture loss
fractions at a given physical radius.
Merging galaxy clusters leave long-lasting signatures on the baryonic and non-baryonic cluster constituents,
including shock fronts, cold fronts, X-ray substructure, radio halos, and offsets between the dark matter (DM) and
the gas components. Using observations from Chandra, the Jansky Very Large Array, the Giant Metrewave Radio
Telescope, and the Hubble Space Telescope, we present a multiwavelength analysis of the merging Frontier Fields
cluster MACS J0416.1-2403 (z = 0.396), which consists of NE and SW subclusters whose cores are separated on
the sky by ∼250 kpc. We find that the NE subcluster has a compact core and hosts an X-ray cavity, yet it is not a
cool core. Approximately 450 kpc south–southwest of the SW subcluster, we detect a density discontinuity that
corresponds to a compression factor of ∼1.5. The discontinuity was most likely caused by the interaction of the
SW subcluster with a less massive structure detected in the lensing maps SW of the subclusterʼs center. For both
the NE and the SW subclusters, the DM and the gas components are well-aligned, suggesting that MACS J0416.1-
2403 is a pre-merging system. The cluster also hosts a radio halo, which is unusual for a pre-merging system. The
halo has a 1.4 GHz power of (1.3 ± 0.3) × 1024WHz−1, which is somewhat lower than expected based on the
X-ray luminosity of the cluster if the spectrum of the halo is not ultra-steep. We suggest that we are either
witnessing the birth of a radio halo, or have discovered a rare ultra-steep spectrum halo.
The Variable Detection of Atmospheric Escape around the Young, Hot Neptune AU...Sérgio Sacani
Photoevaporation is a potential explanation for several features within exoplanet demographics. Atmospheric escape
observed in young Neptune-sized exoplanets can provide insight into and characterize which mechanisms drive this
evolution and at what times they dominate. AU Mic b is one such exoplanet, slightly larger than Neptune (4.19 R⊕).
It closely orbits a 23 Myr pre-main-sequence M dwarf with an orbital period of 8.46 days. We obtained two visits of
AU Mic b at Lyα with Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph. One flare within the
first HST visit is characterized and removed from our search for a planetary transit. We present a nondetection in our
first visit, followed by the detection of escaping neutral hydrogen ahead of the planet in our second visit. The outflow
absorbed ∼30% of the star’s Lyα blue wing 2.5 hr before the planet’s white-light transit. We estimate that the
highest-velocity escaping material has a column density of 1013.96 cm−2 and is moving 61.26 km s−1 away from the
host star. AU Mic b’s large high-energy irradiation could photoionize its escaping neutral hydrogen in 44 minutes,
rendering it temporarily unobservable. Our time-variable Lyα transit ahead of AU Mic b could also be explained by
an intermediate stellar wind strength from AU Mic that shapes the escaping material into a leading tail. Future Lyα
observations of this system will confirm and characterize the unique variable nature of its Lyα transit, which,
combined with modeling, will tune the importance of stellar wind and photoionization.
Is the atmosphere of the ultra-hot Jupiter WASP-121 b variable?Sérgio Sacani
We present a comprehensive analysis of the Hubble Space Telescope observations of the atmosphere
of WASP-121 b, a ultra-hot Jupiter. After reducing the transit, eclipse, and phase-curve observations
with a uniform methodology and addressing the biases from instrument systematics, sophisticated
atmospheric retrievals are used to extract robust constraints on the thermal structure, chemistry, and
cloud properties of the atmosphere. Our analysis shows that the observations are consistent with a
strong thermal inversion beginning at ∼104 Pa on the dayside, solar to subsolar metallicity Z (i.e.,
−0.77 < log(Z) < 0.05), and super-solar C/O ratio (i.e., 0.59 < C/O < 0.87). More importantly,
utilizing the high signal-to-noise ratio and repeated observations of the planet, we identify the following
unambiguous time-varying signals in the data: i) a shift of the putative hotspot offset between the
two phase-curves and ii) varying spectral signatures in the transits and eclipses. By simulating the
global dynamics of WASP-121 b atmosphere at high-resolution, we show that the identified signals are
consistent with quasi-periodic weather patterns, hence atmospheric variability, with signatures at the
level probed by the observations (∼5% to ∼10%) that change on a timescale of ∼5 planet days; in
the simulations, the weather patterns arise from the formation and movement of storms and fronts,
causing hot (as well as cold) patches of atmosphere to deform, separate, and mix in time.
Search for an Isotropic Gravitational-wave Background with the Parkes Pulsar ...Sérgio Sacani
Pulsar timing arrays aim to detect nanohertz-frequency gravitational waves (GWs). A background of GWs
modulates pulsar arrival times and manifests as a stochastic process, common to all pulsars, with a signature spatial
correlation. Here we describe a search for an isotropic stochastic gravitational-wave background (GWB) using
observations of 30 millisecond pulsars from the third data release of the Parkes Pulsar Timing Array (PPTA), which
spans 18 yr. Using current Bayesian inference techniques we recover and characterize a common-spectrum noise
process. Represented as a strain spectrum h Af c 1yr = -1 a ( ) , we measure A 3.1 10 0.9 = ´ 1.3 15 -
+ - and
α = −0.45 ± 0.20, respectively (median and 68% credible interval). For a spectral index of α = −2/3,
corresponding to an isotropic background of GWs radiated by inspiraling supermassive black hole binaries, we
recover an amplitude of A 2.04 10 0.22 = ´ 0.25 15 -
+ - . However, we demonstrate that the apparent signal strength is timedependent, as the first half of our data set can be used to place an upper limit on A that is in tension with the inferred
common-spectrum amplitude using the complete data set. We search for spatial correlations in the observations by
hierarchically analyzing individual pulsar pairs, which also allows for significance validation through randomizing
pulsar positions on the sky. For a process with α = −2/3, we measure spatial correlations consistent with a GWB,
with an estimated false-alarm probability of p 0.02 (approx. 2σ). The long timing baselines of the PPTA and the
access to southern pulsars will continue to play an important role in the International Pulsar Timing Array.
First Direct Imaging of a Kelvin–Helmholtz Instability by PSP/WISPRSérgio Sacani
We present a comprehensive analysis aimed at proving the hypothesis that a train of small-scale features observed by the Wide-field Imager (WISPR) onboard the Parker Solar Probe (PSP) are the signature of a Kelvin–Helmholtz instability (KHI). These features were seen near the flank of a Coronal Mass Ejection (CME) wake between 7.5 Re and 9.5Re, lasting for about 30 minutes. The CME was a slow event, associated with a streamer blowout. We analyzed the size of the eddies and found growth during their evolution while maintaining separation distances and alignment typical of Kelvin–Helmholtz vortexes. We then assessed the magnetic field conditions that would make the observation of such an instability plausible. Two methods were used to cross-check our findings. The measured thickness of the boundary layer supports KHI candidacy, and the estimated linear growth rate suggests nonlinear saturation within the expected timescale. We conclude that a KHI is a plausible explanation for the observed features, and therefore that such instabilities might exist in the low and middle solar corona (within ∼15 Re) and can be detected in white light observations. Their observation, however, might be rare due to stringent conditions like the observer’s proximity, suitable viewing circumstances, magnetic field topology, and flow properties. This study highlights the unique capability of PSP/WISPR in observing such phenomena, especially as PSP perihelia reach closer distances to the Sun.
Is Betelgeuse Really Rotating? Synthetic ALMA Observations of Large-scale Con...Sérgio Sacani
The evolved stages of massive stars are poorly understood, but invaluable constraints can be derived from spatially resolved observations of nearby red supergiants, such as Betelgeuse. Atacama Large Millimeter/submillimeter Array (ALMA) observations of Betelgeuse showing a dipolar velocity field have been interpreted as evidence for a projected rotation rate of about 5 km s−1. This is 2 orders of magnitude larger than predicted by single-star evolution, which led to suggestions that Betelgeuse is a binary merger. We propose instead that large-scale convective motions can mimic rotation, especially if they are only partially resolved. We support this claim with 3D CO5BOLDsimulations of nonrotating red supergiants that we postprocessed to predict ALMA images and SiO spectra. We show that our synthetic radial velocity maps have a 90% chance of being falsely interpreted as evidence for a projected rotation rate of 2 km s−1 or larger for our fiducial simulation. We conclude that we need at least another ALMA observation to firmly establish whether Betelgeuse is indeed rapidly rotating. Such observations would also provide insight into the role of angular momentum and binary interaction in the late evolutionary stages. The data will further probe the structure and complex physical processes in the atmospheres of red supergiants, which are immediate progenitors of supernovae and are believed to be essential in the formation of gravitational-wave sources.
Similar to A Spatially Resolved Analysis of Star Formation Burstiness by Comparing UV and Hα in Galaxies at z∼1 with UVCANDELS (20)
EWOCS-I: The catalog of X-ray sources in Westerlund 1 from the Extended Weste...Sérgio Sacani
Context. With a mass exceeding several 104 M⊙ and a rich and dense population of massive stars, supermassive young star clusters
represent the most massive star-forming environment that is dominated by the feedback from massive stars and gravitational interactions
among stars.
Aims. In this paper we present the Extended Westerlund 1 and 2 Open Clusters Survey (EWOCS) project, which aims to investigate
the influence of the starburst environment on the formation of stars and planets, and on the evolution of both low and high mass stars.
The primary targets of this project are Westerlund 1 and 2, the closest supermassive star clusters to the Sun.
Methods. The project is based primarily on recent observations conducted with the Chandra and JWST observatories. Specifically,
the Chandra survey of Westerlund 1 consists of 36 new ACIS-I observations, nearly co-pointed, for a total exposure time of 1 Msec.
Additionally, we included 8 archival Chandra/ACIS-S observations. This paper presents the resulting catalog of X-ray sources within
and around Westerlund 1. Sources were detected by combining various existing methods, and photon extraction and source validation
were carried out using the ACIS-Extract software.
Results. The EWOCS X-ray catalog comprises 5963 validated sources out of the 9420 initially provided to ACIS-Extract, reaching a
photon flux threshold of approximately 2 × 10−8 photons cm−2
s
−1
. The X-ray sources exhibit a highly concentrated spatial distribution,
with 1075 sources located within the central 1 arcmin. We have successfully detected X-ray emissions from 126 out of the 166 known
massive stars of the cluster, and we have collected over 71 000 photons from the magnetar CXO J164710.20-455217.
The debris of the ‘last major merger’ is dynamically youngSérgio Sacani
The Milky Way’s (MW) inner stellar halo contains an [Fe/H]-rich component with highly eccentric orbits, often referred to as the
‘last major merger.’ Hypotheses for the origin of this component include Gaia-Sausage/Enceladus (GSE), where the progenitor
collided with the MW proto-disc 8–11 Gyr ago, and the Virgo Radial Merger (VRM), where the progenitor collided with the
MW disc within the last 3 Gyr. These two scenarios make different predictions about observable structure in local phase space,
because the morphology of debris depends on how long it has had to phase mix. The recently identified phase-space folds in Gaia
DR3 have positive caustic velocities, making them fundamentally different than the phase-mixed chevrons found in simulations
at late times. Roughly 20 per cent of the stars in the prograde local stellar halo are associated with the observed caustics. Based
on a simple phase-mixing model, the observed number of caustics are consistent with a merger that occurred 1–2 Gyr ago.
We also compare the observed phase-space distribution to FIRE-2 Latte simulations of GSE-like mergers, using a quantitative
measurement of phase mixing (2D causticality). The observed local phase-space distribution best matches the simulated data
1–2 Gyr after collision, and certainly not later than 3 Gyr. This is further evidence that the progenitor of the ‘last major merger’
did not collide with the MW proto-disc at early times, as is thought for the GSE, but instead collided with the MW disc within
the last few Gyr, consistent with the body of work surrounding the VRM.
Observation of Io’s Resurfacing via Plume Deposition Using Ground-based Adapt...Sérgio Sacani
Since volcanic activity was first discovered on Io from Voyager images in 1979, changes
on Io’s surface have been monitored from both spacecraft and ground-based telescopes.
Here, we present the highest spatial resolution images of Io ever obtained from a groundbased telescope. These images, acquired by the SHARK-VIS instrument on the Large
Binocular Telescope, show evidence of a major resurfacing event on Io’s trailing hemisphere. When compared to the most recent spacecraft images, the SHARK-VIS images
show that a plume deposit from a powerful eruption at Pillan Patera has covered part
of the long-lived Pele plume deposit. Although this type of resurfacing event may be common on Io, few have been detected due to the rarity of spacecraft visits and the previously low spatial resolution available from Earth-based telescopes. The SHARK-VIS instrument ushers in a new era of high resolution imaging of Io’s surface using adaptive
optics at visible wavelengths.
Earliest Galaxies in the JADES Origins Field: Luminosity Function and Cosmic ...Sérgio Sacani
We characterize the earliest galaxy population in the JADES Origins Field (JOF), the deepest
imaging field observed with JWST. We make use of the ancillary Hubble optical images (5 filters
spanning 0.4−0.9µm) and novel JWST images with 14 filters spanning 0.8−5µm, including 7 mediumband filters, and reaching total exposure times of up to 46 hours per filter. We combine all our data
at > 2.3µm to construct an ultradeep image, reaching as deep as ≈ 31.4 AB mag in the stack and
30.3-31.0 AB mag (5σ, r = 0.1” circular aperture) in individual filters. We measure photometric
redshifts and use robust selection criteria to identify a sample of eight galaxy candidates at redshifts
z = 11.5 − 15. These objects show compact half-light radii of R1/2 ∼ 50 − 200pc, stellar masses of
M⋆ ∼ 107−108M⊙, and star-formation rates of SFR ∼ 0.1−1 M⊙ yr−1
. Our search finds no candidates
at 15 < z < 20, placing upper limits at these redshifts. We develop a forward modeling approach to
infer the properties of the evolving luminosity function without binning in redshift or luminosity that
marginalizes over the photometric redshift uncertainty of our candidate galaxies and incorporates the
impact of non-detections. We find a z = 12 luminosity function in good agreement with prior results,
and that the luminosity function normalization and UV luminosity density decline by a factor of ∼ 2.5
from z = 12 to z = 14. We discuss the possible implications of our results in the context of theoretical
models for evolution of the dark matter halo mass function.
THE IMPORTANCE OF MARTIAN ATMOSPHERE SAMPLE RETURN.Sérgio Sacani
The return of a sample of near-surface atmosphere from Mars would facilitate answers to several first-order science questions surrounding the formation and evolution of the planet. One of the important aspects of terrestrial planet formation in general is the role that primary atmospheres played in influencing the chemistry and structure of the planets and their antecedents. Studies of the martian atmosphere can be used to investigate the role of a primary atmosphere in its history. Atmosphere samples would also inform our understanding of the near-surface chemistry of the planet, and ultimately the prospects for life. High-precision isotopic analyses of constituent gases are needed to address these questions, requiring that the analyses are made on returned samples rather than in situ.
Multi-source connectivity as the driver of solar wind variability in the heli...Sérgio Sacani
The ambient solar wind that flls the heliosphere originates from multiple
sources in the solar corona and is highly structured. It is often described
as high-speed, relatively homogeneous, plasma streams from coronal
holes and slow-speed, highly variable, streams whose source regions are
under debate. A key goal of ESA/NASA’s Solar Orbiter mission is to identify
solar wind sources and understand what drives the complexity seen in the
heliosphere. By combining magnetic feld modelling and spectroscopic
techniques with high-resolution observations and measurements, we show
that the solar wind variability detected in situ by Solar Orbiter in March
2022 is driven by spatio-temporal changes in the magnetic connectivity to
multiple sources in the solar atmosphere. The magnetic feld footpoints
connected to the spacecraft moved from the boundaries of a coronal hole
to one active region (12961) and then across to another region (12957). This
is refected in the in situ measurements, which show the transition from fast
to highly Alfvénic then to slow solar wind that is disrupted by the arrival of
a coronal mass ejection. Our results describe solar wind variability at 0.5 au
but are applicable to near-Earth observatories.
Gliese 12 b: A Temperate Earth-sized Planet at 12 pc Ideal for Atmospheric Tr...Sérgio Sacani
Recent discoveries of Earth-sized planets transiting nearby M dwarfs have made it possible to characterize the
atmospheres of terrestrial planets via follow-up spectroscopic observations. However, the number of such planets
receiving low insolation is still small, limiting our ability to understand the diversity of the atmospheric
composition and climates of temperate terrestrial planets. We report the discovery of an Earth-sized planet
transiting the nearby (12 pc) inactive M3.0 dwarf Gliese 12 (TOI-6251) with an orbital period (Porb) of 12.76 days.
The planet, Gliese 12 b, was initially identified as a candidate with an ambiguous Porb from TESS data. We
confirmed the transit signal and Porb using ground-based photometry with MuSCAT2 and MuSCAT3, and
validated the planetary nature of the signal using high-resolution images from Gemini/NIRI and Keck/NIRC2 as
well as radial velocity (RV) measurements from the InfraRed Doppler instrument on the Subaru 8.2 m telescope
and from CARMENES on the CAHA 3.5 m telescope. X-ray observations with XMM-Newton showed the host
star is inactive, with an X-ray-to-bolometric luminosity ratio of log 5.7 L L X bol » - . Joint analysis of the light
curves and RV measurements revealed that Gliese 12 b has a radius of 0.96 ± 0.05 R⊕,a3σ mass upper limit of
3.9 M⊕, and an equilibrium temperature of 315 ± 6 K assuming zero albedo. The transmission spectroscopy metric
(TSM) value of Gliese 12 b is close to the TSM values of the TRAPPIST-1 planets, adding Gliese 12 b to the small
list of potentially terrestrial, temperate planets amenable to atmospheric characterization with JWST.
Gliese 12 b, a temperate Earth-sized planet at 12 parsecs discovered with TES...Sérgio Sacani
We report on the discovery of Gliese 12 b, the nearest transiting temperate, Earth-sized planet found to date. Gliese 12 is a
bright (V = 12.6 mag, K = 7.8 mag) metal-poor M4V star only 12.162 ± 0.005 pc away from the Solar system with one of the
lowest stellar activity levels known for M-dwarfs. A planet candidate was detected by TESS based on only 3 transits in sectors
42, 43, and 57, with an ambiguity in the orbital period due to observational gaps. We performed follow-up transit observations
with CHEOPS and ground-based photometry with MINERVA-Australis, SPECULOOS, and Purple Mountain Observatory,
as well as further TESS observations in sector 70. We statistically validate Gliese 12 b as a planet with an orbital period of
12.76144 ± 0.00006 d and a radius of 1.0 ± 0.1 R⊕, resulting in an equilibrium temperature of ∼315 K. Gliese 12 b has excellent
future prospects for precise mass measurement, which may inform how planetary internal structure is affected by the stellar
compositional environment. Gliese 12 b also represents one of the best targets to study whether Earth-like planets orbiting cool
stars can retain their atmospheres, a crucial step to advance our understanding of habitability on Earth and across the galaxy.
The importance of continents, oceans and plate tectonics for the evolution of...Sérgio Sacani
Within the uncertainties of involved astronomical and biological parameters, the Drake Equation
typically predicts that there should be many exoplanets in our galaxy hosting active, communicative
civilizations (ACCs). These optimistic calculations are however not supported by evidence, which is
often referred to as the Fermi Paradox. Here, we elaborate on this long-standing enigma by showing
the importance of planetary tectonic style for biological evolution. We summarize growing evidence
that a prolonged transition from Mesoproterozoic active single lid tectonics (1.6 to 1.0 Ga) to modern
plate tectonics occurred in the Neoproterozoic Era (1.0 to 0.541 Ga), which dramatically accelerated
emergence and evolution of complex species. We further suggest that both continents and oceans
are required for ACCs because early evolution of simple life must happen in water but late evolution
of advanced life capable of creating technology must happen on land. We resolve the Fermi Paradox
(1) by adding two additional terms to the Drake Equation: foc
(the fraction of habitable exoplanets
with significant continents and oceans) and fpt
(the fraction of habitable exoplanets with significant
continents and oceans that have had plate tectonics operating for at least 0.5 Ga); and (2) by
demonstrating that the product of foc
and fpt
is very small (< 0.00003–0.002). We propose that the lack
of evidence for ACCs reflects the scarcity of long-lived plate tectonics and/or continents and oceans on
exoplanets with primitive life.
A Giant Impact Origin for the First Subduction on EarthSérgio Sacani
Hadean zircons provide a potential record of Earth's earliest subduction 4.3 billion years ago. Itremains enigmatic how subduction could be initiated so soon after the presumably Moon‐forming giant impact(MGI). Earlier studies found an increase in Earth's core‐mantle boundary (CMB) temperature due to theaccumulation of the impactor's core, and our recent work shows Earth's lower mantle remains largely solid, withsome of the impactor's mantle potentially surviving as the large low‐shear velocity provinces (LLSVPs). Here,we show that a hot post‐impact CMB drives the initiation of strong mantle plumes that can induce subductioninitiation ∼200 Myr after the MGI. 2D and 3D thermomechanical computations show that a high CMBtemperature is the primary factor triggering early subduction, with enrichment of heat‐producing elements inLLSVPs as another potential factor. The models link the earliest subduction to the MGI with implications forunderstanding the diverse tectonic regimes of rocky planets.
Climate extremes likely to drive land mammal extinction during next supercont...Sérgio Sacani
Mammals have dominated Earth for approximately 55 Myr thanks to their
adaptations and resilience to warming and cooling during the Cenozoic. All
life will eventually perish in a runaway greenhouse once absorbed solar
radiation exceeds the emission of thermal radiation in several billions of
years. However, conditions rendering the Earth naturally inhospitable to
mammals may develop sooner because of long-term processes linked to
plate tectonics (short-term perturbations are not considered here). In
~250 Myr, all continents will converge to form Earth’s next supercontinent,
Pangea Ultima. A natural consequence of the creation and decay of Pangea
Ultima will be extremes in pCO2 due to changes in volcanic rifting and
outgassing. Here we show that increased pCO2, solar energy (F⨀;
approximately +2.5% W m−2 greater than today) and continentality (larger
range in temperatures away from the ocean) lead to increasing warming
hostile to mammalian life. We assess their impact on mammalian
physiological limits (dry bulb, wet bulb and Humidex heat stress indicators)
as well as a planetary habitability index. Given mammals’ continued survival,
predicted background pCO2 levels of 410–816 ppm combined with increased
F⨀ will probably lead to a climate tipping point and their mass extinction.
The results also highlight how global landmass configuration, pCO2 and F⨀
play a critical role in planetary habitability.
Constraints on Neutrino Natal Kicks from Black-Hole Binary VFTS 243Sérgio Sacani
The recently reported observation of VFTS 243 is the first example of a massive black-hole binary
system with negligible binary interaction following black-hole formation. The black-hole mass (≈10M⊙)
and near-circular orbit (e ≈ 0.02) of VFTS 243 suggest that the progenitor star experienced complete
collapse, with energy-momentum being lost predominantly through neutrinos. VFTS 243 enables us to
constrain the natal kick and neutrino-emission asymmetry during black-hole formation. At 68% confidence
level, the natal kick velocity (mass decrement) is ≲10 km=s (≲1.0M⊙), with a full probability distribution
that peaks when ≈0.3M⊙ were ejected, presumably in neutrinos, and the black hole experienced a natal
kick of 4 km=s. The neutrino-emission asymmetry is ≲4%, with best fit values of ∼0–0.2%. Such a small
neutrino natal kick accompanying black-hole formation is in agreement with theoretical predictions.
Detectability of Solar Panels as a TechnosignatureSérgio Sacani
In this work, we assess the potential detectability of solar panels made of silicon on an Earth-like
exoplanet as a potential technosignature. Silicon-based photovoltaic cells have high reflectance in the
UV-VIS and in the near-IR, within the wavelength range of a space-based flagship mission concept
like the Habitable Worlds Observatory (HWO). Assuming that only solar energy is used to provide
the 2022 human energy needs with a land cover of ∼ 2.4%, and projecting the future energy demand
assuming various growth-rate scenarios, we assess the detectability with an 8 m HWO-like telescope.
Assuming the most favorable viewing orientation, and focusing on the strong absorption edge in the
ultraviolet-to-visible (0.34 − 0.52 µm), we find that several 100s of hours of observation time is needed
to reach a SNR of 5 for an Earth-like planet around a Sun-like star at 10pc, even with a solar panel
coverage of ∼ 23% land coverage of a future Earth. We discuss the necessity of concepts like Kardeshev
Type I/II civilizations and Dyson spheres, which would aim to harness vast amounts of energy. Even
with much larger populations than today, the total energy use of human civilization would be orders of
magnitude below the threshold for causing direct thermal heating or reaching the scale of a Kardashev
Type I civilization. Any extraterrrestrial civilization that likewise achieves sustainable population
levels may also find a limit on its need to expand, which suggests that a galaxy-spanning civilization
as imagined in the Fermi paradox may not exist.
Jet reorientation in central galaxies of clusters and groups: insights from V...Sérgio Sacani
Recent observations of galaxy clusters and groups with misalignments between their central AGN jets
and X-ray cavities, or with multiple misaligned cavities, have raised concerns about the jet – bubble
connection in cooling cores, and the processes responsible for jet realignment. To investigate the
frequency and causes of such misalignments, we construct a sample of 16 cool core galaxy clusters and
groups. Using VLBA radio data we measure the parsec-scale position angle of the jets, and compare
it with the position angle of the X-ray cavities detected in Chandra data. Using the overall sample
and selected subsets, we consistently find that there is a 30% – 38% chance to find a misalignment
larger than ∆Ψ = 45◦ when observing a cluster/group with a detected jet and at least one cavity. We
determine that projection may account for an apparently large ∆Ψ only in a fraction of objects (∼35%),
and given that gas dynamical disturbances (as sloshing) are found in both aligned and misaligned
systems, we exclude environmental perturbation as the main driver of cavity – jet misalignment.
Moreover, we find that large misalignments (up to ∼ 90◦
) are favored over smaller ones (45◦ ≤ ∆Ψ ≤
70◦
), and that the change in jet direction can occur on timescales between one and a few tens of Myr.
We conclude that misalignments are more likely related to actual reorientation of the jet axis, and we
discuss several engine-based mechanisms that may cause these dramatic changes.
The solar dynamo begins near the surfaceSérgio Sacani
The magnetic dynamo cycle of the Sun features a distinct pattern: a propagating
region of sunspot emergence appears around 30° latitude and vanishes near the
equator every 11 years (ref. 1). Moreover, longitudinal flows called torsional oscillations
closely shadow sunspot migration, undoubtedly sharing a common cause2. Contrary
to theories suggesting deep origins of these phenomena, helioseismology pinpoints
low-latitude torsional oscillations to the outer 5–10% of the Sun, the near-surface
shear layer3,4. Within this zone, inwardly increasing differential rotation coupled with
a poloidal magnetic field strongly implicates the magneto-rotational instability5,6,
prominent in accretion-disk theory and observed in laboratory experiments7.
Together, these two facts prompt the general question: whether the solar dynamo is
possibly a near-surface instability. Here we report strong affirmative evidence in stark
contrast to traditional models8 focusing on the deeper tachocline. Simple analytic
estimates show that the near-surface magneto-rotational instability better explains
the spatiotemporal scales of the torsional oscillations and inferred subsurface
magnetic field amplitudes9. State-of-the-art numerical simulations corroborate these
estimates and reproduce hemispherical magnetic current helicity laws10. The dynamo
resulting from a well-understood near-surface phenomenon improves prospects
for accurate predictions of full magnetic cycles and space weather, affecting the
electromagnetic infrastructure of Earth.
Extensive Pollution of Uranus and Neptune’s Atmospheres by Upsweep of Icy Mat...Sérgio Sacani
In the Nice model of solar system formation, Uranus and Neptune undergo an orbital upheaval,
sweeping through a planetesimal disk. The region of the disk from which material is accreted by
the ice giants during this phase of their evolution has not previously been identified. We perform
direct N-body orbital simulations of the four giant planets to determine the amount and origin of solid
accretion during this orbital upheaval. We find that the ice giants undergo an extreme bombardment
event, with collision rates as much as ∼3 per hour assuming km-sized planetesimals, increasing the
total planet mass by up to ∼0.35%. In all cases, the initially outermost ice giant experiences the
largest total enhancement. We determine that for some plausible planetesimal properties, the resulting
atmospheric enrichment could potentially produce sufficient latent heat to alter the planetary cooling
timescale according to existing models. Our findings suggest that substantial accretion during this
phase of planetary evolution may have been sufficient to impact the atmospheric composition and
thermal evolution of the ice giants, motivating future work on the fate of deposited solid material.
Emergent ribozyme behaviors in oxychlorine brines indicate a unique niche for...Sérgio Sacani
Mars is a particularly attractive candidate among known astronomical objects
to potentially host life. Results from space exploration missions have provided
insights into Martian geochemistry that indicate oxychlorine species, particularly perchlorate, are ubiquitous features of the Martian geochemical landscape. Perchlorate presents potential obstacles for known forms of life due to
its toxicity. However, it can also provide potential benefits, such as producing
brines by deliquescence, like those thought to exist on present-day Mars. Here
we show perchlorate brines support folding and catalysis of functional RNAs,
while inactivating representative protein enzymes. Additionally, we show
perchlorate and other oxychlorine species enable ribozyme functions,
including homeostasis-like regulatory behavior and ribozyme-catalyzed
chlorination of organic molecules. We suggest nucleic acids are uniquely wellsuited to hypersaline Martian environments. Furthermore, Martian near- or
subsurface oxychlorine brines, and brines found in potential lifeforms, could
provide a unique niche for biomolecular evolution.
Continuum emission from within the plunging region of black hole discsSérgio Sacani
The thermal continuum emission observed from accreting black holes across X-ray bands has the potential to be leveraged as a
powerful probe of the mass and spin of the central black hole. The vast majority of existing ‘continuum fitting’ models neglect
emission sourced at and within the innermost stable circular orbit (ISCO) of the black hole. Numerical simulations, however,
find non-zero emission sourced from these regions. In this work, we extend existing techniques by including the emission
sourced from within the plunging region, utilizing new analytical models that reproduce the properties of numerical accretion
simulations. We show that in general the neglected intra-ISCO emission produces a hot-and-small quasi-blackbody component,
but can also produce a weak power-law tail for more extreme parameter regions. A similar hot-and-small blackbody component
has been added in by hand in an ad hoc manner to previous analyses of X-ray binary spectra. We show that the X-ray spectrum
of MAXI J1820+070 in a soft-state outburst is extremely well described by a full Kerr black hole disc, while conventional
models that neglect intra-ISCO emission are unable to reproduce the data. We believe this represents the first robust detection of
intra-ISCO emission in the literature, and allows additional constraints to be placed on the MAXI J1820 + 070 black hole spin
which must be low a• < 0.5 to allow a detectable intra-ISCO region. Emission from within the ISCO is the dominant emission
component in the MAXI J1820 + 070 spectrum between 6 and 10 keV, highlighting the necessity of including this region. Our
continuum fitting model is made publicly available.
WASP-69b’s Escaping Envelope Is Confined to a Tail Extending at Least 7 RpSérgio Sacani
Studying the escaping atmospheres of highly irradiated exoplanets is critical for understanding the physical
mechanisms that shape the demographics of close-in planets. A number of planetary outflows have been observed
as excess H/He absorption during/after transit. Such an outflow has been observed for WASP-69b by multiple
groups that disagree on the geometry and velocity structure of the outflow. Here, we report the detection of this
planet’s outflow using Keck/NIRSPEC for the first time. We observed the outflow 1.28 hr after egress until the
target set, demonstrating the outflow extends at least 5.8 × 105 km or 7.5 Rp This detection is significantly longer
than previous observations, which report an outflow extending ∼2.2 planet radii just 1 yr prior. The outflow is
blueshifted by −23 km s−1 in the planetary rest frame. We estimate a current mass-loss rate of 1 M⊕ Gyr−1
. Our
observations are most consistent with an outflow that is strongly sculpted by ram pressure from the stellar wind.
However, potential variability in the outflow could be due to time-varying interactions with the stellar wind or
differences in instrumental precision.
Nutraceutical market, scope and growth: Herbal drug technologyLokesh Patil
As consumer awareness of health and wellness rises, the nutraceutical market—which includes goods like functional meals, drinks, and dietary supplements that provide health advantages beyond basic nutrition—is growing significantly. As healthcare expenses rise, the population ages, and people want natural and preventative health solutions more and more, this industry is increasing quickly. Further driving market expansion are product formulation innovations and the use of cutting-edge technology for customized nutrition. With its worldwide reach, the nutraceutical industry is expected to keep growing and provide significant chances for research and investment in a number of categories, including vitamins, minerals, probiotics, and herbal supplements.
Nucleophilic Addition of carbonyl compounds.pptxSSR02
Nucleophilic addition is the most important reaction of carbonyls. Not just aldehydes and ketones, but also carboxylic acid derivatives in general.
Carbonyls undergo addition reactions with a large range of nucleophiles.
Comparing the relative basicity of the nucleophile and the product is extremely helpful in determining how reversible the addition reaction is. Reactions with Grignards and hydrides are irreversible. Reactions with weak bases like halides and carboxylates generally don’t happen.
Electronic effects (inductive effects, electron donation) have a large impact on reactivity.
Large groups adjacent to the carbonyl will slow the rate of reaction.
Neutral nucleophiles can also add to carbonyls, although their additions are generally slower and more reversible. Acid catalysis is sometimes employed to increase the rate of addition.
hematic appreciation test is a psychological assessment tool used to measure an individual's appreciation and understanding of specific themes or topics. This test helps to evaluate an individual's ability to connect different ideas and concepts within a given theme, as well as their overall comprehension and interpretation skills. The results of the test can provide valuable insights into an individual's cognitive abilities, creativity, and critical thinking skills
This presentation explores a brief idea about the structural and functional attributes of nucleotides, the structure and function of genetic materials along with the impact of UV rays and pH upon them.
Comparing Evolved Extractive Text Summary Scores of Bidirectional Encoder Rep...University of Maribor
Slides from:
11th International Conference on Electrical, Electronics and Computer Engineering (IcETRAN), Niš, 3-6 June 2024
Track: Artificial Intelligence
https://www.etran.rs/2024/en/home-english/
ESR spectroscopy in liquid food and beverages.pptxPRIYANKA PATEL
With increasing population, people need to rely on packaged food stuffs. Packaging of food materials requires the preservation of food. There are various methods for the treatment of food to preserve them and irradiation treatment of food is one of them. It is the most common and the most harmless method for the food preservation as it does not alter the necessary micronutrients of food materials. Although irradiated food doesn’t cause any harm to the human health but still the quality assessment of food is required to provide consumers with necessary information about the food. ESR spectroscopy is the most sophisticated way to investigate the quality of the food and the free radicals induced during the processing of the food. ESR spin trapping technique is useful for the detection of highly unstable radicals in the food. The antioxidant capability of liquid food and beverages in mainly performed by spin trapping technique.
BREEDING METHODS FOR DISEASE RESISTANCE.pptxRASHMI M G
Plant breeding for disease resistance is a strategy to reduce crop losses caused by disease. Plants have an innate immune system that allows them to recognize pathogens and provide resistance. However, breeding for long-lasting resistance often involves combining multiple resistance genes
What is greenhouse gasses and how many gasses are there to affect the Earth.moosaasad1975
What are greenhouse gasses how they affect the earth and its environment what is the future of the environment and earth how the weather and the climate effects.
2. primarily impact the low-mass scales (Springel 2005; Fontanot
et al. 2009; Governato et al. 2010; Hopkins et al. 2014;
Somerville & Davé 2015). One of the manifestations of these
stellar feedback effects is stochasticity in the star formation
activity, often referred to as “bursty” star formation. Conse-
quently, quantifying this star formation burstiness from
observations of galaxies provides key constraints for theoretical
models.
Some spectral features are particularly sensitive to ongoing
star formation, such as rest-frame UV, IR, submillimeter, and
radio continua, as well as nebular emission lines (e.g., Hα),
which have been used extensively as star formation indicators.
Rest-frame UV and Hα are two widely used star formation
indicators. However, these two tracers are sensitive to star
formation occurring over different timescales: the Hα recom-
bination line originates from H II regions powered by the hot O-
and B-type stars and decays quickly after ∼5 Myr, whereas the
longer-lived B- and A-type stars are capable of emitting
significant amounts of rest-UV continuum over ∼200 Myr
timescales. Standard calibrations that infer star formation rates
(SFRs) from rest-UV or Hα luminosity (e.g., Kennicutt &
Evans 2012) assume that those SFRs do not change
significantly on ∼200 Myr timescales, but approximate an
equilibrium state. However, if the SFR varies on timescales
shorter than ∼200 Myr, then the rest-UV and Hα luminosities
also vary in response to the variations in star formation and, in
these cases, the standard calibrations break down.
Several studies have used the flux ratio of the rest-UV and
Hα luminosities (e.g., Glazebrook et al. 1999; Iglesias-Páramo
et al. 2004; Lee et al. 2009, 2011; Weisz et al. 2012;
Kauffmann 2014; Domínguez et al. 2015; Mehta et al. 2017;
Emami et al. 2019; Fetherolf et al. 2021) as well as the scatter
in the two luminosities (e.g., Broussard et al. 2019, 2022; Faisst
et al. 2019; Griffiths et al. 2021) to infer the burstiness in the
star formation histories (SFHs) of galaxies. There are a number
of factors that can affect the UV-to-Hα ratio, such as dust,
stellar initial mass functions (IMFs), stellar metallicity, as well
as the escape of ionizing photons and the inclusion of binaries
and rotating stars in stellar models. However, after accounting
for dust, variations in the SFH have the largest impact on the
UV-to-Hα ratio.
Existing observational studies that have used the comparison
of rest-UV and Hα to infer the burstiness in galaxy star
formation have relied on integrated flux measurements and thus
are limited to global galaxy properties (e.g., Lee et al. 2009;
Weisz et al. 2012; Guo et al. 2016; Emami et al. 2019). In this
analysis, we use high-resolution imaging as well as slitless
spectroscopy from the Hubble Space Telescope (HST) to
perform a resolved analysis of these two star formation
indicators. The combination of rest-UV imaging from the
UltraViolet imaging of the Cosmic Assembly Near-infrared
Deep Extragalactic Legacy Survey Fields (UVCANDELS; X.
Wang et al. 2023, in preparation) and near-IR slitless
spectroscopy from the 3D-HST (Brammer et al. 2012; Skelton
et al. 2014; Momcheva et al. 2016) and A Grism H-Alpha
SpecTroscopic (AGHAST; Weiner 2009) surveys over four of
the CANDELS (Grogin et al. 2011; Koekemoer et al. 2011)
fields (GOODS-N, GOODS-S, COSMOS, and EGS) has
provided resolved imaging of both rest-frame UV as well as
the Hα line emission for galaxies at 0.7 < z < 1.5 and thus
enabled a unique opportunity for a spatially resolved invest-
igation of burstiness by comparing the two star formation
indicators. In this work, our primary goal is to use the UV-to-
Hα flux ratio to infer the burstiness in galaxy SFHs and
quantify its evolution as a function of galaxy structural
parameters.
This manuscript is organized as follows: Section 2 describes
the UVCANDELS and 3D-HST+AGHAST data sets as well
as the selection criteria for the galaxy sample used for this
analysis; Section 3 describes the data processing performed in
order to obtain calibrated and matched maps for the rest-UV
and Hα emission for our sample; Section 4 outlines how the
comparison of two star formation indicators helps us infer
details about the SFH; Section 5 presents our stacking analysis
along with the results; Section 6 discusses the impacts of
various key assumptions made through the analysis as well as
the implications of our results in the context of galaxy SFHs;
and Section 7 summarizes our conclusions.
Throughout this paper, we adopt cosmological parameters
from Table 3 of Planck Collaboration et al. (2016):
Ωm = 0.315, Ωλ = 0.685, and H0 = 67.31 km s−1
Mpc−1
, and
all magnitudes used are AB magnitudes (Oke & Gunn 1983).
2. Data and Sample Selection
The galaxy sample used for the analysis presented in this
work is obtained from the recently completed UVCANDELS
Hubble Treasury program.
2.1. UVCANDELS Imaging
UVCANDELS (PI: H. Teplitz; PID: 15647; X. Wang et al.
2023, in preparation) is a Cycle 26 Hubble Treasury program
that obtained UV imaging of four of the deep-wide survey
fields defined by CANDELS (Grogin et al. 2011; Koekemoer
et al. 2011). UVCANDELS consists of Wide Field Camera 3
(WFC3)/F275W imaging over a total combined area of
∼430 arcmin2
over four CANDELS fields (GOODS-N,
GOODS-S, COSMOS, and EGS), reaching an imaging depth
of AB = 27 for compact galaxies, along with Advanced
Camera for Surveys (ACS)/F435W (reaching AB 28.0)
imaging in parallel. The UVCANDELS F275W mosaics are
available at the Mikulski Archive for Space Telescopes
(doi:10.17909/8s31-f778)20
for all four fields, along with
F435W mosaics for COSMOS and EGS.21
These UVCAN-
DELS image mosaics are aligned and registered to the
CANDELS astrometry. With the newly acquired UVCAN-
DELS imaging, we have measured the fluxes in the F275W
filter for the four fields, along with the deeper F435W for
COSMOS and EGS. We adopt a similar methodology as
Rafelski et al. (2015), where F275W flux measurements rely on
isophotes defined on the F606W mosaics optimizing the signal-
to-noise ratio (S/N) for the near-UV photometry of small faint
sources, followed by an aperture correction to match it to the
H-band isophote-based photometry for the optical and near-IR
bands. The full description of the UVCANDELS data set,
including the mosaics as well as photometry, will be provided
in X. Wang et al. (2023, in preparation). The stellar physical
properties for these galaxies as estimated from spectral energy
distribution (SED) fitting will be available from V. Mehta et al.
(2023, in preparation; but see Section 2.3 for a brief
description).
20
https://archive.stsci.edu/hlsp/uvcandels
21
The F435W mosaics for GOODS-N and GOODS-S are already available
from CANDELS.
2
The Astrophysical Journal, 952:133 (17pp), 2023 August 1 Mehta et al.
3. The galaxy sample used in the analysis presented in this
work is selected from the UVCANDELS photometric catalogs.
The full sample selection is described in Section 2.3. We also
use the image mosaics generated from UVCANDELS for our
resolved analysis, since the F275W filter allows us to directly
probe the rest-frame UV light from galaxies over our redshift
range of interest, 0.7 < z < 1.5. Furthermore, we also use the
F435W mosaics22
for deriving resolved dust maps (see
Section 3.3), as well as the F160W mosaics from CANDELS23
to ensure accurate astrometric alignment (see Section 3.1).
2.2. 3D-HST+AGHAST Grism Spectroscopy
We use the archival slitless grism spectroscopy data from the
3D-HST (Brammer et al. 2012; Skelton et al. 2014; Momcheva
et al. 2016) and AGHAST (Weiner 2009) surveys for
measuring the Hα emission for our galaxy sample. The 3D-
HST+AGHAST program is a widefield, near-IR grism
spectroscopy, HST-based survey that obtained WFC3/G141
grism coverage for ∼500 arcmin2
over all five CANDELS
fields. In this work, we use the recent reanalysis of this archival
HST grism data in the CANDELS fields carried out as part of
the Complete Hubble Archive for Galaxy Evolution (CHArGE)
initiative (see Kokorev et al. 2022).
Briefly, this reduction of 3D-HST+AGHAST data uses the
state-of-the-art GRIZLI
24
software (Brammer & Matharu 2021),
which executes an iterative forward-modeling approach to
perform quantitative fitting of the spectral trace for each object
in the field-of-view of the grism exposures at the visit level.
This approach also includes a comprehensive modeling of
contamination due to nearby objects and their own spectral
traces. As its final science-ready data products, GRIZLI
provides cutouts of the direct image and segmentation maps
used for modeling the object, as well as the 1D/2D grism
spectra and continuum-subtracted emission line maps (see, e.g.,
Wang et al. 2022). In this analysis, we use the Hα emission line
maps as well as a cutout of the F140W mosaic, which serves as
a direct image for the grism spectral extraction.
2.3. Sample Selection
The galaxy sample for our resolved analysis of rest-frame
UV and Hα emission is selected to include all galaxies with a
significant detection in the UVCANDELS F275W images as
well as in their measured Hα fluxes. Specifically, we require
the S/N to be >5 for both the measured integrated F275W
flux and the integrated Hα emission line flux. The
wavelength coverage of the G141 grism (1.1−1.7 μm)
establishes the redshift range for our sample as 0.7 < z < 1.5,
where it covers the rest-frame Hα. Over this redshift range,
the F275W filter probes the rest-frame ∼1100–1600 Å light,
thus giving access to the second star formation indicator, the
rest-UV.
Before finalizing our galaxy sample, we remove any objects
that do not have F435W coverage,25
which is required for our
dust measurement (see Section 3.3 for details). We also exclude
any potential AGN candidates from the deep Chandra X-ray
observations in these CANDELS fields (Hsu et al. 2014;
Nandra et al. 2015; Xue et al. 2016; D. Kocevski 2023, private
communication). Furthermore, we perform a round of visual
vetting to exclude any objects that are affected by data quality
issues or incorrect redshift estimates due to line misidentifica-
tion, source confusion, trace contamination, or other artifacts.
While the Hα and [N II] doublet are not resolved in the G141
grism spectra, the other emission feature that is most common
for our sample, i.e., the Hβ +[O III] complex, is marginally
resolved and is easily identified. Our visual vetting step serves
as an additional check to ensure the sample purity; however,
the GRIZLI redshift estimate from the combined fit to the grism
spectrum and photometry is robust and we do not find any
objects with misidentified emission lines.
Our final sample consists of 979 galaxies in the redshift
range of 0.7 < z < 1.5 over four CANDELS fields (GOODS-N,
GOODS-S, COSMOS, and EGS). Figure 1 shows a few
examples handpicked to have high S/N as well as extended
morphologies, in order to demonstrate the variation between
the rest-UV and Hα emission. These examples are not
representative of our full galaxy sample, as a majority of the
objects are rather compact and not as morphologically
impressive.
2.4. Estimating the Stellar Physical Properties
The spectral information from 3D-HST+AGHAST grism
data provides precise redshifts for our sample and the stellar
physical properties are computed using these grism redshifts.
Briefly, the stellar population properties are measured on
integrated photometry using the DENSE BASIS code (Iyer
et al. 2019), which is based on the Flexible Stellar Population
Synthesis (FSPS) library (Conroy et al. 2009; Conroy &
Gunn 2010) and utilizes Gaussian processes to parameterize
flexible SFHs. We define the DENSE BASIS template grid
with a set of free parameters: stellar metallicity ( Z Z
log( )
varying over −1.5 to 0.25), dust (AV varying from 0 to 4),
and four “shape” (tX) parameters to define the SFHs. We
assume the Chabrier (2003) IMF and Calzetti et al. (2000)
dust law for the model templates. DENSE BASIS outputs the
model template that best fits the observed data, along with the
estimated stellar mass, SFR, and other stellar population
properties. Figure 2 shows the redshift and stellar mass
distributions for our final sample. These distributions
illustrate that our sample uniformly spans the redshift range
0.7 < z < 1.5. The sample includes galaxies that range in
mass from 108−11.5
Me, roughly corresponding to the
present-day Small Magellanic Cloud (SMC) at the low-mass
end to the Milky Way at the high-mass end.
3. Resolved Maps of Rest-frame UV and Hα Emission
We start with the final data products available from the
GRIZLI 3D-HST+AGHAST reduction, which includes an
8″ × 8″ (at 0 1 pixel scale) continuum-subtracted emission
map for all detected lines and image cutouts in the direct
images (typically, F140W). We then extract a matching cutout
from the UVCANDELS F275W mosaic (at 0 06 pixel scale),
using SWARP,26
which allows us to adjust the pixel scale as
22
We use the mosaics from UVCANDELS for the COSMOS and EGS fields,
and the existing ones from CANDELS for the GOODS-N and GOODS-S
fields.
23
https://archive.stsci.edu/hlsp/candels
24
https://github.com/gbrammer/grizli
25
The coverage in F435W has some gaps, due to the nature of the parallel
observation setup for simultaneous F275W and F435W imaging. This is a
geometrical selection that does not introduce biases for our analysis.
26
https://github.com/astromatic/swarp
3
The Astrophysical Journal, 952:133 (17pp), 2023 August 1 Mehta et al.
4. Figure 1. A few handpicked examples of the extended galaxies in our sample with their (from left to right) F140W, rest-frame UV, Hα, and UV-to-Hα ratio maps.
While the majority of our sample consists of objects that are more compact, these selected examples explicitly highlight the spatial variation between the rest-UV and
Hα emission. The stamps shown are all 8″ × 8″ (at 0 1 pixel scale). See Section 3 for details on how the maps are generated. Some of the bright features (not
associated with the galaxy) visible in the Hα stamps are either emission lines from chance alignments with spectral traces from other nearby sources in the grism
observations, hot pixels, or artifacts from data reduction in the Hα stamps that typically appear at the edge of the grism cutout. The UV-to-Hα ratio maps for individual
galaxies are shown here purely as a visual aid; our analysis and results rely on first stacking the rest-UV and Hα emission (see Section 5.1 for details).
4
The Astrophysical Journal, 952:133 (17pp), 2023 August 1 Mehta et al.
5. well as image orientation to match that of the Hα maps from
GRIZLI.
3.1. Aligning UVCANDELS and 3D-HST+AGHAST
The astrometric reference frames used to align the
UVCANDELS mosaics and the GRIZLI reduction of the 3D-
HST+AGHAST data, which are used for this work, are not
consistent. Since we are primarily interested in object-specific
cutouts for our analysis, we need to account for any
misalignment on an object-by-object basis when making their
cutouts.
We measure the astrometric offset from a cross-correlation
of the images. In order to minimize the impact of galaxy
morphology at different wavelengths, we use the CANDELS
F160W mosaic, which has an identical astrometric reference as
UVCANDELS and is the closest match in wavelength
coverage to the F140W direct images from 3D-HST
+AGHAST. Additionally, we mask all objects other than the
target galaxy to ensure there are no mismatches. Finally, we
spatially cross-correlate the CANDELS F160W cutout with the
3D-HST+AGHAST F140W from GRIZLI to quantify the
astrometric offset between the two data sets. The astrometric
offsets vary from field to field, but are within ∼0 2 in R.A. and
within ∼0 1 in decl. for our sample. We apply the corrections
necessary for proper alignment when extracting the final
cutouts used for further analysis.
3.2. Calibrating the Maps
The aligned and matched F275W and Hα emission maps are
corrected for residual background by performing a local back-
ground estimation and subtracting it from the stamps. The maps
are then calibrated to account for instrument and filter response
and converted into luminosity units (erg s−1
Å−1
kpc−2
for rest-
UV; and erg s−1
kpc−2
for Hα).
The maps are further corrected for galactic extinction using the
Schlegel et al. (1998) dust maps.27
The reddening E(B − V ) is
queried at the location of each galaxy and converted to an
extinction, assuming a Cardelli et al. (1989) extinction law, for
the respective filters as well as for Hα at its observed
wavelength.
Since the Hα and [N II] λλ6548 + 6584 doublet are blended
at the G141 resolution, we need to apply a correction to the Hα
emission maps to remove the [N II] contribution. The [N II]/Hα
ratio in galaxies has been shown to vary by up to factors of 3–5
and this variation has been linked to various galaxy properties,
such as metallicity, stellar mass, SFR, and nitrogen-to-oxygen
ratio, as well as known to evolve as a function of redshift (e.g.,
Kewley et al. 2013; Masters et al. 2014, 2016; Steidel et al.
2014; Shapley et al. 2015; Strom et al. 2017; Faisst et al. 2018).
Here, we use the mass- and redshift-dependent calibration from
Faisst et al. (2018) to apply the appropriate correction for each
galaxy according to its redshift and best-fit stellar mass.
3.3. Correcting for Dust
Before being interpreted as star formation indicators, both
rest-UV and Hα emission need to be corrected for attenuation
due to the presence of dust in the host galaxy itself. Since the
main goal of this work is to compare the rest-UV and Hα
emission from galaxies as a function of their positions inside
the galaxy, it is imperative to make an effort to address the fact
that dust absorption may itself vary inside a galaxy (e.g., Kim
et al. 2019). Hence, we compute a resolved dust map for each
galaxy to correct for dust attenuation, while preserving the
morphological detail.
In order to compute the resolved dust map, we use the
relation between the UV slope (β; Calzetti et al. 1994) and the
dust attenuation (AV). We rely on imaging in the F275W,
F435W, and F606W filters in order to constrain β. This process
is less precise than SED fitting, but performing a (pixel-by-
pixel) resolved SED fit is beyond the scope of this work. We
estimate β assuming the functional form fλ ∝ λβ
, using
photometry from the filters closest to the rest-frame near-UV.
For objects with redshifts z 1.08, we use the F275W and
F435W filters, whereas for objects with z 1.08, where the
F275W filter falls below rest-frame 1300 Å, we use the F435W
and F606W filters.28
The measurement of β with this methodology is unreliable
for low-S/N pixels. For each galaxy where the individual
pixels fall below an S/N of 3, we instead use a default value for
β that is computed from the total integrated flux for the galaxy
in the relevant filters. This ensures that β is measured
consistently and reliably for all pixels. As a sanity check, we
confirm that the UV slopes measured from the two-band
integrated photometry for the full galaxy are in good agreement
with those computed from the best-fit SED models.
With the maps of UV slope β computed for our sample, we
then use the β–AV relation to convert to a resolved map of dust
attenuation, AV. Specifically, we use the calibration from Reddy
et al. (2018), assuming the Calzetti et al. (2000) dust law.29
We
confirm that only 12%, 9%, and 4% of the per-pixel β values
and only 0.2% of the β values computed from integrated galaxy
Figure 2. The redshift and stellar mass distribution of our final sample of
979 galaxies over 0.7 < z < 1.5 with rest-UV coverage available from
UVCANDELS and Hα from 3D-HST+AGHAST. The symbols are colored
according to their effective radius (Reff,50) as measured in the F160W filter (as
reported in the CANDELS photometric catalogs), converted to physical units.
The typical 1σ error bar for the best-fit stellar masses is shown in the lower left
corner.
27
Specifically, we use the Python implementation available at https://github.
com/adrn/SFD to query the SFD dust maps.
28
Where F606W imaging is available.
29
β = − 2.616 + 4.684 × E(B − V ).
5
The Astrophysical Journal, 952:133 (17pp), 2023 August 1 Mehta et al.
6. photometry fall below the dust-free β0 = − 2.616 value implied
for the Reddy et al. (2018) model galaxy templates. Given that
measurement noise, particularly for low-S/N pixels, is likely to
drive the extended tails of the β distribution, we believe the
models from Reddy et al. (2018; with β0 = − 2.616) are not
inappropriate for our sample. Figure 3 shows an example of a
resolved dust map generated following the steps outlined here.
Last, we use this AV map to apply a dust correction to the rest-
UV and Hα maps based on the Calzetti et al. (2000) dust law,
assuming E(B − V )stellar/E(B − V )nebular = 0.44 (Calzetti et al.
2000). For the values computed from the integrated galaxy
flux, the median β for our sample is −1.17 with an interquantile
range (IQR) spanning over −1.53 to −0.78, whereas the
median E(B − V )stellar value is 0.3 with an IQR of 0.22 to 0.38.
3.4. Matching the Point-spread Function
The point-spread function (PSF) evolves considerably
between the near-IR (G141, which covers Hα) and F275W
(which provides rest-UV). In order to be able to draw accurate
comparisons between the spatial extent of rest-UV and Hα, it is
imperative to correct for the change in the PSF. We address this
issue by convolving the F275W maps to match the near-IR
PSF. The target PSF is selected from WFC3/IR F105W,
F125W, and F160W, whichever is the closest match (in
wavelength) to the observed wavelength of the Hα emission.
We then use PYPHER30
(Boucaud et al. 2016) to compute a
convolution kernel that, when applied, converts the F275W
PSF to that of the Hα emission.
4. Using Rest-UV and Hα to Infer Burstiness
The rest-frame UV continuum and the Hα recombination
line are both indicators of ongoing star formation activity in
galaxies (Kennicutt et al. 1994; Kennicutt & Evans 2012).
However, these indicators are sensitive to star formation
activity over different timescales. The Hα recombination line
originates in the H II regions surrounding the hot O- and B-type
stars and it decays rapidly ∼5 Myr after a cessation of star
formation. On the other hand, the longer-lived B- and A-type
stars continue to power the total rest-frame UV continuum over
a ∼200 Myr timescale. This difference in the timescales traced
by rest-UV and Hα enables us to probe the star formation
activity in galaxies over the recent ∼200 Myr.
Several studies have performed comparisons across these
two star formation indicators to infer the intrinsic properties of
the galaxy SFHs in observational surveys (e.g., Glazebrook
et al. 1999; Iglesias-Páramo et al. 2004; Boselli et al. 2009; Lee
et al. 2009, 2011, 2012; Weisz et al. 2012; Guo et al. 2016;
Mehta et al. 2017; Emami et al. 2019; Broussard et al. 2022) as
well as in simulations (Shen et al. 2013; Hopkins et al. 2014;
Domínguez et al. 2015; Broussard et al. 2019; Emami et al.
2021; Griffiths et al. 2021). These studies have typically
compared the ratio of the luminosities of the two indicators or
its scatter to infer the galaxy SFH. However, there are several
other effects that also affect the ratio of the observed rest-UV
and Hα luminosities from galaxies:
1. Dust attenuation: the attenuation of a galaxy spectrum
due to dust is strongly dependent on wavelength.
Moreover, the spatial distribution of the dust can be
nonuniform, where the dust content attenuating the light
from the stellar birth clouds is different compared to that
attenuating the nebular emission (Kewley et al. 2002; Lee
et al. 2009).
2. IMF: the ratio of the rest-UV and Hα luminosities is
sensitive to the relative number of stars at each mass and
hence is affected by the IMF. There are two main scenarios
to consider, both of which manifest at small (low-SFR)
scales. (i) a nonuniversal IMF, such as the integrated
galactic IMF (IGIMF; Weidner & Kroupa 2005;
Pflamm-Altenburg et al. 2007, 2009; Weidner et al.
2011), could lead to a steeper IMF slope at low-SFR values
(<10−2
Me yr−1
; Pflamm-Altenburg et al. 2009), due to a
lower chance of forming mass stars relative to the high-SFR
case. (ii) A universal IMF may not fully sample at all SFRs
and could be biased against the very massive stars. Low-
SFR galaxies may not be able to properly sample the high-
mass end of the IMF, leading to stochasticity on a star
cluster scale (e.g., Fumagalli et al. 2011; da Silva et al.
2012; Eldridge 2012; Cerviño 2013). However, the impact
from this stochastic IMF sampling, by itself, has been
shown to be smaller compared to that of the IGIMF case
(Fumagalli et al. 2011; Andrews et al. 2013, 2014).
3. Stellar metallicity: the stellar abundance affects the stars’
temperatures, where stars with lower metal abundances
are hotter and, thus, the Hα emission is intensified
relative to the UV, resulting in a lower value of the UV-
to-Hα ratio compared to their higher-metallicity
Figure 3. The resolved dust map for an example galaxy computed via the UV
slope β estimated from the F275W and F435W images and the Reddy et al.
(2018) calibration of the the β–AV relation (see the text for details). The final
dust map consists of independent AV measurements for individual pixels with
sufficient S/N and a default value computed from the total integrated galaxy
flux in F275W and F435W for pixels with S/N < 3.
30
https://pypher.readthedocs.io/
6
The Astrophysical Journal, 952:133 (17pp), 2023 August 1 Mehta et al.
7. counterparts; however, this effect is minimal (∼10%;
Bicker & Fritze-v. Alvensleben 2005; Boselli et al. 2009).
4. Other effects, such as the escape of ionizing photons as
well as the inclusion of binary (Eldridge 2012) and
rotating stars (Choi et al. 2017) in modeling stellar
populations, can also impact the UV-to-Hα ratio.
While all of these factors affect the observed ratio of the rest-
UV and Hα luminosities, the impact of dust attenuation and the
SFH is the most dominant. However, attenuation due to dust is
well parameterized with the help of dust laws (e.g., Calzetti
et al. 2000) and the dust content of galaxies can be measured
independently. Hence, we can correct the ratio of the observed
rest-UV and Hα luminosities for dust and use it to infer the
SFHs of galaxies. For the sake of convenience, we introduce a
“burstiness” parameter (ζ) in the rest of our analysis, which is
defined as the dust-corrected UV-to-Hα luminosity ratio:
z n
= n a
L L
log . 1
,UV H
( ) ( )
Figure 4 demonstrates the evolution of ζ under different
assumptions of SFHs, stellar IMFs, and metallicities using
theoretical galaxy model templates. We use the FSPS library
(Conroy et al. 2009; Conroy & Gunn 2010) with the MILES
stellar library and MIST31
isochrones to generate the SED of
our mock stellar populations. We compute the rest-UV
(1500 Å) luminosity density and Hα line luminosities for a
range of stellar populations to track the evolution of ζ over a
10 Gyr timescale. We vary the IMF for the stellar population
among Chabrier (2003), Salpeter (1955), and Kroupa (2001).
The stellar metallicity is varied over a range of
Z Z
log( )
= −2 to 0 and the gas-phase metallicity is set to
be equal to the stellar. We compare star formation bursts of
varying durations and burst shapes with the constant star
formation scenario. In our toy model, the starbursts are
introduced on top of a low level of underlying constant star
formation at the level of ∼0.1% of the peak burst amplitude,
which is consistent with the SFHs inferred for ∼108−10
Me
galaxies observed in the local universe (e.g., Weisz et al. 2012;
Kauffmann 2014; Emami et al. 2019). Last, since we are
considering the dust-free luminosity ratio (ζ), there is no dust
added to our toy model.
The evolution of ζ in an idealistic scenario of individual
bursts of star formation is shown in the left panel of Figure 4. In
the case of constant star formation, the value of ζ asymptotes to
an equilibrium value after ∼100–200 Myr. Accounting for
variations in IMF and stellar metallicities, the equilibrium value
for ζ spans a narrow range of ∼60–85. However, in the case of
bursts, ζ quickly deviates from its constant star formation
value, both during the rise and fall of star formation. Initially,
during the rise, ζ decreases due to the increasingly bright Hα
emission from the hot O- and B-type stars. Toward the end of
the burst, once the star formation starts to decline, the value of ζ
increases significantly (by several dex), as the O- and B-type
stars die off and the Hα emission starts to decay, while the rest-
UV continuum survives for a longer duration. Ultimately, the
strength of both the rest-UV and Hα emission from the
starburst decay to the point where their contribution relative to
low-level, underlying constant star formation is not significant
and the value of ζ returns to its equilibrium value. The general
trend in the time evolution of ζ is reproduced regardless of the
metallicities. Variations due to the choice of fiducial IMFs are
on the scale of the line width of the curves drawn in Figure 4
(left) and are therefore negligible. Varying the upper mass
cutoff (Mup) for the IMF can mimic an increase in the value of
ζ. In the case of the Chabrier (2003) IMF, changing Mup from
its default value (100 Me) to 80, 60, 40, and 20 Me causes the
equilibrium ζ value to rise from 90 to 100, 140, 240, and 1640,
respectively. However, it is important to caution that tweaking
the Mup value in this way is rather arbitrary; instead, the IGIMF
provides a more physically motivated formulation of this effect.
The right panel of Figure 4 instead illustrates the behavior of
ζ in the case of periodic short bursts that are again introduced
atop a low, underlying baseline. The stellar IMF is fixed to be
Chabrier (2003) and the stellar metallicity is fixed to solar for
this case. The value of ζ responds to the individual bursts,
similar to the earlier case, but its time evolution is now
Figure 4. The time evolution of the dust-corrected UV-to-Hα ratio (ζ), as computed from stellar population synthesis models (see the text for details on the models).
Left: an idealistic scenario showing SFHs with a single burst of varying duration (1, 10, 50, and 100 Myr, shown in purple, blue, green, and cyan curves, respectively)
atop a low level of underlying star formation. In the constant star formation case (red curve), the value of ζ reaches an equilibrium value (shown as the gray hatched,
shaded band) within ∼200 Myr, whereas in the burst cases, the value of ζ quickly deviates significantly from its equilibrium state after the burst ends. Right: an
idealistic scenario showing the behavior of ζ in the case of multiple bursts of varying shapes. The value of ζ is driven below that of the equilibrium state (the gray
hatched, shaded region; the same as in the left panel) during the rising phase of the burst, and above the equilibrium during the declining phase. The SFHs with sharper
evolution (top hat in blue and exponentially peaked in red) result in a sharper change in ζ compared to the smoother sinusoidal (in green) bursts. On the other hand, a
long-term smooth rise/decline in SFR (shown in black) is also able to reproduce similar deviations in the ζ value as the short-term bursts.
31
http://waps.cfa.harvard.edu/MIST/
7
The Astrophysical Journal, 952:133 (17pp), 2023 August 1 Mehta et al.
8. truncated by the onset of new starbursts, which dominate the
relative contribution to the intrinsic rest-UV and Hα luminos-
ities. The deviation in ζ from the equilibrium value retains its
asymmetrical behavior, where ζ deviates significantly more (by
up to several dex, depending on the strength of the burst) after a
burst, compared to during the burst onset/rise. Thus, provided
that the star formation is not constant and the duration of the
bursts is on the order of the timescales between two
consecutive bursts or shorter, the observed value of ζ, on
average, will be systematically higher than the equilibrium
value for constant star formation. The exact shape of the burst
has minimal impact on the behavior of ζ, as is evident from
Figure 4. The change in ζ is only slightly exaggerated in the
case of sharply changing SFHs (the exponentially peaking and
top-hat cases) compared to the sinusoidal case.
However, it is important to mention that smoothly rising or
falling SFHs can also result in a similar behavior in ζ. A long-
term smooth decline (rise) in SFR also causes ζ to be
systematically higher (lower) than the equilibrium value, as
illustrated in Figure 4 (right). This case is similar to having a
single smoothly evolving burst and the rate of change in SFR is
the primary factor that controls the deviation in ζ. Nonetheless,
long-term smoothly rising/falling SFHs can also be responsible
for the observed ζ value, similar to the case with multiple short-
term bursts.
5. Results
5.1. Rest-UV and Hα Light Profiles via Stacking
The main goal of this work is to study the spatially resolved
evolution of burstiness in galaxy SFHs. With the resolved maps
of rest-UV and Hα emission in hand (from Section 3), we now
focus on computing the burstiness parameter (ζ, Section 4) as a
function of galaxy structural parameters, namely galactocentric
radius and stellar mass surface density. On an individual galaxy
basis, variations in ζ can be rather large, due to several galaxy
property–dependent factors. We marginalize over these to
calculate an average value of ζ, which can then provide an
insight into the average properties of the general galaxy
population. In this work, we adopt a stacking approach to
compute an azimuthally averaged ζ for our sample of
979 galaxies.
To do this, we start with the rest-UV and Hα maps computed
from Section 3 that have been properly aligned, calibrated, and
corrected for dust and matched for PSF variations. Further-
more, we also process the F140W image stamps from 3D-HST
+AGHAST for each galaxy through the same steps. We use
the F140W stamps, which trace the rest-frame optical light for
our sample, to define a centroid that is not as strongly affected
by recent star formation activity and gives a more robust galaxy
centroid. For each galaxy, we then bin all pixels belonging to
the galaxy32
according to their galactocentric radius. We define
the galactocentric radius as the distance from the centroid
normalized by the effective radius33
(Reff,50) in F160W in
CANDELS photometric catalogs. After binning the rest-UV
and Hα emission maps for all galaxies in our sample, we then
take the median for each radius bin to compute the average rest-
UV and Hα 1D light profiles. We opt for a median over a mean
when stacking, to ensure that the light profiles are not biased by
the brightest objects in the sample.
Figure 5 (top) shows the average light profiles for our full
sample, along with the total number of galaxies and pixels in
each bin of radius shown in the inset. All pixels that fall within
the galaxies’ segmentation map are considered when taking the
median, regardless of the per-pixel S/N. This allows us to
naturally recover the noise properties of our data by measuring
the error on the median (shown as the shaded confidence
regions in Figure 9), and we exclude any bins where the light
profile is not well constrained (i.e., where both rest-UV and Hα
stacked average light profiles are measured at >1σ and at least
>100 galaxies are contributing to the stack in the corresp-
onding bin). We do not apply any normalization to the
individual galaxies’ light profiles when stacking, since we want
to compute the UV-to-Hα flux ratio, ζ, and need to preserve the
inherent variations in the total flux.
5.2. Average ζ as a Function of Galactocentric Radius
We compute the average burstiness parameter, ζ, from the
rest-UV and Hα light profiles as a function of galactocentric
radius (shown in the bottom panel of Figure 5). When
computing the average ζ, we only consider bins where the
measured median values of both rest-UV and Hα light profiles
are significant (>1σ). As is evident from the figure, ζ shows a
clear trend as a function of galactocentric radius. The average ζ
near the galactic centers (R/Reff < 0.5) is broadly consistent
with the equilibrium value expected from constant star
formation. On the other hand, as we move toward the outskirts
(R/Reff > 1), the average ζ trends upward, deviating from the
equilibrium value.
The stellar mass of a galaxy is one of the key physical
quantities that is linked to its overall star formation activity (e.g.,
Speagle et al. 2014). Hence, we extend our analysis to investigate
the radial trend of ζ as a function of the galaxy stellar mass.
Owing to the sensitivity and area of UVCANDELS and 3D-HST
+AGHAST, our sample of 979 galaxies is large enough to allow
us to split it into subsamples of mass and still have sufficient
number statistics for our analysis. We use the stellar mass
estimated from SED fitting (Section 2.3) to split our sample into
three bins: low mass (<109.5
Me; 372 galaxies), intermediate mass
(109.5−10.2
Me; 313 galaxies), and high mass (>1010.2
Me;
294 galaxies).
Figure 6 shows the averaged ζ as a function of galactocentric
radius now split according to the three mass bins. There is a
clear and significant trend in the radius-dependent slope of ζ as
a function of the stellar mass. We fit a linear slope to the inner
region (R/Reff < 1.8) where the trend in ζ is most well
constrained. We find best-fit slopes of −0.01 ± 0.09,
0.02 ± 0.08, and 0.22 ± 0.10 for the low-, intermediate-, and
high-mass bins, respectively (also shown in the inset of
Figure 6), and 0.11 ± 0.05 for the full sample. We find a
dispersion in ζ of 0.1−0.8 dex per bin in galactocentric radius
(0.2 Reff,50), with an increasingly larger dispersion toward
larger radii. Given our light profiles (and ζ trends) are
computed out to Reff,50 of ∼3, these dispersion values are
comparable to those reported in recent literature for integrated
galaxy fluxes (e.g., 0.06−0.16 dex from Broussard et al. 2022).
We discuss the implications of this result for the galaxy SFHs
in Sections 6.2.1 and 6.2.2.
32
For rest-UV, the segmentation map is defined using the F435W images, as
per the UVCANDELS photometry (X. Wang et al. 2023, in preparation);
whereas for Hα, it is defined as per GRIZLI, using the F140W direct imaging
that was obtained alongside the grism images.
33
The circular radius that encloses 50% of the total galaxy flux.
8
The Astrophysical Journal, 952:133 (17pp), 2023 August 1 Mehta et al.
9. 5.3. Average ζ as a Function of Surface Brightness
It is well established that the local stellar mass content (i.e.,
the mass surface density of existing stellar populations) is a key
factor for regulating star formation on galaxy scales (e.g.,
Kennicutt 1998; Kennicutt & Evans 2012) and, consequently, it
is likely that the local SFH within the galaxy is also impacted
by the stellar mass surface density.
While the mass surface density of galaxies generally falls off
as a function of radius, the morphology of star-forming
galaxies can often be irregular, especially at the redshifts of our
sample (e.g., Scarlata et al. 2007; Patel et al. 2013; Shibuya
et al. 2016) and particularly at wavelengths that trace ongoing
star formation activity. The azimuthally averaged ζ as a
function of radius (as in Section 5.2) marginalizes over key
morphological features with active star formation, e.g., star-
forming clumps or spiral arms. Instead of the galactocentric
radius, the local stellar mass surface density is a more direct
tracer of the local environment in the galaxy at the sites of star
formation activity. Ideally, computing the stellar mass surface
density would require a full-scale, multiwavelength, resolved
SED fitting, which is beyond the scope of this work. Instead,
we utilize the surface brightness as a proxy that directly traces
the stellar mass surface density.
We use the calibrated F140W stamps (the same as the ones
used for morphological measurements in Section 5.1) for our
sample after correcting for surface brightness dimming. The
final stamps used for our analysis are units of ABmag kpc−2
.
The F140W filter traces optical wavelengths (5500–8300 Å) at
the redshifts of our sample and thus is minimally sensitive to
the ongoing star formation and a good tracer of the existing
stellar mass. As demonstrated in Dai et al. (2021), the H-band
magnitude and galaxy stellar mass are strongly correlated for
star-forming galaxies over the redshift range of our sample.
Using the empirical relation from Dai et al. (2021),34
we can
Figure 6. The average UV-to-Hα ratio (ζ) plotted as a function of the
galactocentric radius for the full sample as well as three subsamples split
according to galaxy stellar mass. The gray hatched, shaded region shows the
equilibrium value of ζ expected for constant star formation. The shaded bands
denote the 1σ error bars. The dashed lines are the best-fit linear slopes fitted at
R/Reff < 1.8. The inset shows the best-fit values of the slopes along with their
1σ uncertainty as a function of the mean galaxy stellar mass of the subsample.
Figure 5. The average light profile as a function of the galactocentric radius for rest-UV and Hα emission (shown in the top panels) as well as the UV-to-Hα ratio (ζ;
shown in the bottom panels) computed for our galaxy sample (see the text for details). The gray hatched, shaded band shows the equilibrium value expected for the
constant star formation case. In addition to our fiducial case (shown in solid curves), which uses resolved dust maps and the Calzetti et al. (2000) dust law, we also
show additional cases for comparison. In the left panel, the dashed curves show the result assuming the SMC (Gordon et al. 2003) dust law and the dotted curves show
that assuming the Buat et al. (2018) dust law instead. In the right panel, the dashed curves show the result if we assumed a uniform dust value for each galaxy and the
dotted curves show the result if we only consider the pixels with a reliable per-pixel dust estimate. The inset shows the total number of galaxies (in black) and pixels
(in green) that fall within each individual bin in radius. The shaded bands show the 1σ error on the light profiles.
34
We apply an average k-correction for our sample when converting surface
brightness to stellar mass surface density when using the Dai et al. (2021) H-
band absolute magnitude (k-corrected) to stellar mass relation.
9
The Astrophysical Journal, 952:133 (17pp), 2023 August 1 Mehta et al.
10. further interpret the F140W surface brightness as a stellar mass
surface density.
We modify the analysis from Section 5.1, binning the pixels
according to the F140W surface brightness. Figure 7 shows
average ζ computed as a function of the F140W surface
brightness for the full sample, along with the three mass
subsamples described in Section 5.2. We again fit linear slopes
to the trend in ζ and find best-fit slope values of −0.07 ± 0.04,
−0.03 ± 0.03, and 0.13 ± 0.03 for the low-, intermediate-, and
high-mass bins, respectively, and 0.05 ± 0.02 for the full
sample. All the slopes are much better constrained and their
evolution as a function of the galaxy stellar mass is much more
pronounced than the case with radius. The implications of these
trends are discussed in Section 6.2.3.
5.4. Average ζ Trends over Redshift
The redshift range around z ∼ 1 is at the tail end of the
cosmic high noon and is a time where the cosmic star formation
density is evolving rapidly. Given the large galaxy sample size
considered in this work, we are able to further split our analysis
into bins of redshift to investigate any potential evolution in the
ζ trends as a function of cosmic time. In order to ensure
sufficient number statistics for all the stellar mass sub-bins, we
split our sample at z = 1.1 into two redshift bins: 0.7 < z < 1.1
and 1.1 < z < 1.5, with sample sizes of 508 and 471 galaxies,
respectively.
Figure 8 shows the main result from our analysis, i.e., the
trend in ζ as a function of galactocentric radius and F140W
surface brightness for the two bins in redshift along with the
three mass subsamples described in Section 5.2. Overall, the ζ
trends are consistent with flat slopes for the higher-redshift
(1.1 < z < 1.5) bin, both as a function of radius as well as
surface brightness. On the other hand, in the lower-redshift
(0.7 < z < 1.1) bin, the slope of the ζ trends get progressively
steeper as a function of galactocentric radius. In the surface
brightness case, the lower-redshift sample exhibits strong
evolution in ζ for the low- and high-mass bins, with
significantly negative and positive slopes for each case,
respectively. The slope values for all subsamples in mass as
well as redshift are reported in Table 1, and we further discuss
the ζ evolution with redshift in Section 6.2.4.
6. Discussion
6.1. Impact of Key Assumptions
Before delving into discussing the implications of our results
from Section 5 in the context of galaxies’ star formation
properties, it is important to consider the impact of various
assumptions made during the analysis.
Dust correction. Our fiducial case uses the resolved dust
maps (described in Section 3.3) along with the Calzetti et al.
(2000) dust law. To assess the impact of these assumptions, we
test two additional scenarios. First, we switch to using the SMC
(Gordon et al. 2003) dust law along with the appropriate
calibration from Reddy et al. (2018). The steeper SMC dust law
causes the dust-corrected light profiles to be fainter overall
(shown in the dashed curves in the left panel of the Figure 5).
We also check for a dust law from Buat et al. (2018) that is
directly calibrated for z ∼ 0.6 − 1.6 galaxies using HST grism
spectroscopy (shown as the dotted curves in the left panel of
Figure 5). In both these cases, the impact of changing the dust
law on ζ is minimal, as it only affects the overall normalization
of the trend in ζ and does not significantly affect its shape.
Second, we evaluate the impact of using resolved dust maps
(from Section 3.3) when correcting for dust attenuation. We
recompute the light profiles assuming a single value of dust for
the whole galaxy, instead of resolved AV maps. This has a
minimal impact on the overall result (shown as the dotted
curves in Figures 5 and 9). The differences from the fiducial
case are most evident in the inner regions, which is to be
expected, since that is where the individual pixels would have
sufficient S/N to facilitate a reliable per-pixel measurement of
the UV slope β. Importantly, even in the inner regions, the
overall light profiles as well as the trend in ζ are minimally
impacted (see also the Appendix for additional figures).
Similarly, we also perform the converse test, evaluating the
impact on the stacked results if we only consider the pixels
where a reliable per-pixel dust measurement is possible. The
impact of this test on the trends in ζ is even more minimal
(shown as the dashed–dotted curves in Figures 5 and 10). The
resulting trends from this case are nearly identical to the
fiducial case, with the primary difference being that the light
profiles have more noise, particularly in the outskirts, due to
there being fewer pixels with high enough S/N for a reliable
per-pixel dust estimate. From these tests, we are confident that
the observed trend in ζ is not driven by systematic biases from
our particular methodology for dust correction. Overall, our
results are robust against our choice of using resolved dust
maps as well as our dust law assumption.
Last, the factor relating stellar and nebular dust attenuation,
f = E(B − V )stellar/E(B − V )nebular, also affects the computed ζ
trends. For our fiducial result, we have assumed a value of
0.44; however, larger values of f have been suggested from
observational studies up to f ∼ 1 (e.g., Puglisi et al. 2016;
Rodríguez-Muñoz et al. 2022). The primary impact of
changing f to larger values is that the trend in ζ is shifted to
higher values, but most importantly the slope of the ζ relation is
not affected significantly, thus having a minimal impact on our
conclusions.
[N II] Correction. In this analysis, we apply the mass- and
redshift-dependent correction from Faisst et al. (2018) when
Figure 7. Similar to Figure 6, but now as a function of the surface brightness in
F140W (corrected for surface brightness dimming), which serves as a proxy for
the mass surface density (shown as a twinned axis on top). The linear slopes are
fit over the F140W surface brightness range of [−18.5, − 15.5].
10
The Astrophysical Journal, 952:133 (17pp), 2023 August 1 Mehta et al.
11. correcting the blended Hα +[N II] flux from the grism
observations. While this accounts for the evolution in the
[N II]/Hα ratio over cosmic time, as well as its variation due to
global galaxy properties, we still assume a single correction
value for the whole galaxy, whereas galaxies are known to have
metallicity gradients, where the outskirts of galaxies tend to
have lower metallicities than their cores (e.g., Jones et al. 2015;
Wang et al. 2020, 2022; Simons et al. 2021; Li et al. 2022). A
metallicity gradient could result in a gradient in the [N II]/Hα
ratio within a galaxy, which in turn would affect ζ. A lower
[N II]/Hα ratio would require a smaller correction and thus
result in a lower value of ζ. However, the metallicity gradients
are expected to typically be on the order of 0.2 dex kpc−1
even at high redshifts (e.g., Wang et al. 2020, 2022). The
change in ζ due to such a gradient is negligible relative to the
observed trends and is well within the 1σ error bars of our
results. While these gradients may play a role in driving the
trends in ζ, it is not the dominant factor, and the trends in our
results are not exclusively driven by the presence of metallicity
gradients in galaxies.
IGIMF. In Section 4, we consider the impacts of various
canonical IMFs that have traditionally been universally applied
to galaxies. On the other hand, a nonuniversal IMF, such as the
IGIMF (Weidner & Kroupa 2005; Pflamm-Altenburg et al.
2007, 2009; Weidner et al. 2011) predicts an IMF slope that
gets steeper than the canonical case with the decreasing total
SFR of galaxies. The IGIMF predicts a lower chance of
forming massive stars in low-SFR galaxies compared to their
higher-SFR counterparts. Due to the lack of massive stars, an
IGIMF results in low-SFR galaxies having a higher value of ζ.
Pflamm-Altenburg et al. (2009) demonstrate the impact of
IGIMF on ζ and find that for galaxy SFRs < 10−2
Me yr−1
,
there is a sharp upturn in ζ under the IGIMF scenario.
However, the SFR values where this effect is most significant
are lower than those considered in our analysis. None of the
galaxies in our sample have SFRs < 10−2
Me yr−1
, with the
majority having SFRs 10−1
Me yr−1
. Even for our fiducial
result of resolved ζ, the rest-UV and Hα light profiles are limited
to a surface brightness of νLν 1041.8
erg s−1
kpc−2
and
LHα 1039.6
erg s−1
kpc−2
, corresponding to an SFR of
10−1.7
Me yr−1
kpc−2
according to the standard (Kennicutt &
Evans 2012) calibration. Considering the total area in each of our
bins (0.2 Reff,50) and the range of Reff,50 for our sample
(1−10.6 kpc with a median of ∼2.5 kpc), the total SFR is
100
Me yr−1
for all bins analyzed in this work, which is
considerably higher than the regime where the effects of an
IGIMF are significant (Pflamm-Altenburg et al. 2009).
Using surface brightness as a proxy for mass surface
density. The young stars in regions with active star formation
will have a nontrivial contribution at all wavelengths, including
the observed H band (rest-frame optical). In the case where the
relative contribution at rest-frame optical wavelengths from the
young stars is comparable to that from the existing stellar
population, the F140W flux may be boosted due to the ongoing
star formation. This effect would be most significant in the less
dense regions of the galaxy, where it would result in an
increased F140W surface brightness relative to the fiducial
assumption of it tracing just the existing stellar population.
Figure 8. Similar to Figures 6 and 7, but now split into two redshift bins: 0.7 < z < 1.1 and 1.1 < z < 1.5.
11
The Astrophysical Journal, 952:133 (17pp), 2023 August 1 Mehta et al.
12. While this is a second-order effect, this scenario is plausible in
the low-mass galaxies or galaxy outskirts, in general, and could
affect our results from stacking as a function of F140W surface
brightness (Section 5.3).
On the other hand, stacking as a function of galactocentric
radius (Section 5.2) is immune to this effect and hence,
comparing the results from Figures 6 and 7, we can confirm
that the impact of this effect is negligible for the intermediate-
and high-mass bins. This effect could be the cause for the slight
negative ζ slope for the low-mass sample as a function of the
F140W surface brightness (Figure 7). However, since we have
already confirmed a flat slope as a function of galactocentric
radius for the low-mass subsample in Figure 6, we do not
anticipate this effect having any further impact other than
potentially flattening the ζ slope as a function of F140W
surface brightness, making it consistent with our result in
Section 5.2.
6.2. Implications on Burstiness in Galaxies’ SFH
6.2.1. Radial Evolution of ζ
As discussed in Section 4, the galaxy SFH is the one of the
dominant factors that drives the deviations in the value of ζ
relative to its equilibrium value from the constant star
formation case. The presence of bursts causes ζ to scatter
asymmetrically about the equilibrium value, where it deviates
to preferentially larger values following a burst. In this
analysis, we have computed the average ζ as a function of
galactocentric radius (Section 5.2) and rest-frame optical
surface brightness (Section 5.3) for our galaxy sample via a
stacking analysis, providing an overall trend in ζ for a
statistically significant sample of galaxies.
In Figure 5, we find that the average ζ from stacking our full
galaxy sample (0.7 < z < 1.5; 108−11.5
Me) has a significant
positive slope as a function of the galactocentric distance,
where ζ trends upwards toward larger distances. Given that this
value of ζ is a statistic averaged across a population of galaxies,
it indicates that a substantial fraction of galaxies (or pixels)
have ζ that is higher than the equilibrium value, suggestive of
the presence of bursts in the galaxies’ SFHs. At face value, the
observed trend in ζ suggests that, on average, the outskirts of
the galaxies in our sample are experiencing an increased
contribution from starbursts relative to their cores.
It is also important to point out that bursts of star formation
are not the only explanation for deviations in ζ. As discussed in
Section 4, long-term smoothly declining (rising) SFHs can also
result in a value of ζ that is higher (lower) than the equilibrium
value. The higher-than-equilibrium ζ values from Figure 5
could also be indicative of a smoothly declining (or recently
declined and still low) SFR. In this case, the observed trend
would suggest a sharper decline in the galaxies’ SFHs in their
outskirts relative to their cores. Given the degeneracy in the
behavior of ζ in these two scenarios, we are unable to
distinguish between them within the context of this analysis
and thus long-term smoothly rising/falling SFHs remain as a
plausible explanation for the trends in ζ presented in this work.
6.2.2. Impact of Galaxy Mass on the Radial Evolution of ζ
In Figure 6, we present the average ζ trends as a function of
galactocentric distance for the three subsamples split according
to the galaxy stellar mass to investigate the impact of the galaxy
mass on the trends in ζ (see also the Appendix for the average
light profiles and ζ profiles for the individual mass bins). For
the low- (<109.5
Me) and intermediate-mass (109.5−10.2
Me)
bins, the average ζ is roughly constant at all radii, albeit at a
level that is elevated above the equilibrium value from constant
star formation. This suggests that the SFHs in these galaxies do
not change significantly as a function of radius. The higher-
than-equilibrium value of ζ also potentially indicates that these
galaxies have a significant contribution from bursts at all radii.
This is consistent with studies that use the integrated galaxy
flux; e.g., Guo et al. (2016), who find, for 0.4 < z < 1, galaxies
below 109
Me to be dominated by bursty SFHs.
On the other hand, the high-mass bin has a steep slope in ζ as
a function of radius, where the interior regions (R/Reff < 1) are
consistent with the equilibrium value, but their outskirts
(R/Reff > 1.5) clearly appear to have a bursty component to
their SFHs. The strong evolution in ζ for the high-mass bin is
largely responsible for driving the overall trend for the full
sample in Figure 5.
Existing studies of high-mass galaxies typically do not find
significant evidence of burstiness in their SFHs (e.g., Lee et al.
2009; Weisz et al. 2012; Guo et al. 2016; Emami et al. 2019);
however, these findings are based on the full integrated
photometry. What we uncover with this work, instead, is that
even for these massive galaxies, the star formation occurring in
their outskirts has a significant burstiness. It is only when taken
as a whole that the brighter central regions dominate the total
flux contribution and cause the overall SFH of the galaxy to
appear consistent with secular star formation with no
significant bursts.
6.2.3. Evolution of ζ as a Function of Surface Brightness
We find the trends in ζ (shown in Figure 7) to be more
tightly correlated with the rest-frame optical surface brightness,
Table 1
Best-fit Slope Parameters for Trends in ζ Presented in This Work
Sample 108−11.5
Me 108−9.5
Me 109.5−10.2
Me 1010.2−11.5
Me
Slope as a function of galactocentric radius
z = 0.7−1.5 0.111 ± 0.054 −0.014 ± 0.087 0.022 ± 0.083 0.224 ± 0.104
z = 0.7−1.1 0.198 ± 0.047 0.028 ± 0.072 0.122 ± 0.086 0.273 ± 0.068
z = 0.1−1.5 −0.019 ± 0.092 −0.052 ± 0.154 −0.056 ± 0.115 0.049 ± 0.145
Slope as a function of F140W surface brightness
z = 0.7−1.5 0.054 ± 0.019 −0.073 ± 0.040 −0.032 ± 0.028 0.126 ± 0.030
z = 0.7−1.1 0.117 ± 0.017 −0.095 ± 0.019 0.006 ± 0.026 0.177 ± 0.025
z = 0.1−1.5 0.012 ± 0.043 −0.056 ± 0.082 −0.056 ± 0.057 0.085 ± 0.084
12
The Astrophysical Journal, 952:133 (17pp), 2023 August 1 Mehta et al.
13. which traces the galaxies’ morphological features compared to
simply the galactocentric distance. In high-mass galaxies, on
average for the full sample, the observed ζ in brighter (and thus
denser) regions (−17 ABmag kpc−2
or 108.4
Me kpc−2
) of
the galaxy is consistent with the equilibrium value from the
constant star formation scenario. This average trend over the
full sample is driven largely by the lower-redshift (z < 1.1)
galaxies (see Section 6.2.4 for further discussion). On the other
hand, for regions that are fainter (less dense), the average ζ
value suggests a significant contribution from bursts to the
SFH. This is similar to the trend from Figure 6, which is
expected, considering that galactocentric radius is a good tracer
of the mass surface density, especially for high-mass galaxies
that tend to have smoother morphologies.
On the other hand, for low-mass galaxies, we find an
opposite trend, such that the burstiness in the SFHs is
intensified toward brighter (higher-density) regions. It is likely
that for these low-mass and often compact galaxies, the bursts
occurring in the brighter (denser) regions are more intense. The
strength of individual starbursts relative to the underlying, low-
level constant SFR affects the amplitude of the deviation in ζ in
the postburst phase. This could present a plausible explanation
for the observed trend, where bursts in the denser regions of
low-mass galaxies are more intense and the resulting stellar
feedback could more easily eject the existing cold gas and/or
prevent/delay accretion in the neighboring less dense regions.
However, as discussed in Section 6.1, for the low-mass sample,
the ongoing star formation could be contributing significantly
to the rest-frame optical surface brightness, thus rendering it
ineffective as a tracer of the existing stellar population. This
effect could also be a potential reason for the slight negative
slope in ζ as a function of surface brightness. Last, we
reconfirm our findings from the radial case that, on average,
low- and intermediate-mass bins have significant contributions
from bursts in their SFHs, regardless of the local surface
brightness.
Most importantly, our analysis of ζ as a function of the
surface brightness leads us toward a seemingly fundamental
observation. From the trends shown in Figure 7, we note that
for any given galaxy, regardless of its stellar mass, local
regions with a mass surface density below ∼107.5
Me kpc−2
appear to have a significant bursty component in their SFHs.
This could be an indication that, at the smallest scales, the local
SFH is independent of the global galaxy properties. This
delimiting value in mass surface density is provided as a rough
estimate and we caution against treating it as exact, since it is
based on an empirical relation between the H-band flux and
stellar mass from Dai et al. (2021), which is defined for
integrated galaxy fluxes/stellar masses, has considerable
scatter, and is not as tightly constrained down to 108
Me.
6.2.4. Evolution of ζ Trends with Redshift
As per Section 5.4, we split our analysis into two redshift
bins: 0.7 < z < 1.1 and 1.1 < z < 1.5, in order to investigate the
evolution in ζ over this critical time period, when the universe
is undergoing a rapid evolution in its cosmic SFR density. As
shown in Figure 8, we find a clear distinction in the ζ trends for
z > 1.1 galaxies. The ζ trends, as a function of both
galactocentric radius as well as surface brightness, are
consistent with being flat for z > 1.1 galaxies. At face value,
this seems to suggest that the star formation properties of the
general star-forming population at z > 1.1 are similar across the
full range of stellar masses from 108
to 1011.5
Me, with them
appearing to have a consistently bursty SFH with seemingly no
dependence on radius or surface brightness.
On the other hand, the z < 1.1 galaxy population appears to
significantly deviate from flat ζ trends, suggesting that there is
indeed an evolution in the star formation properties within
individual galaxies toward the end of the cosmic high noon. As
the average cosmic star formation activity is declining, the
central (denser) regions of galaxies appear to settle into more
secular star formation activity, whereas the outskirt (less dense)
regions continue to experience bursts in their SFHs, as they did
during the peak of cosmic high noon. Last, splitting the analysis
in redshift also reveals that the ζ trends observed for the full
galaxy sample (see the discussions in Sections 6.2.1 and 6.2.2)
are primarily driven by the low-redshift (z < 1.1) galaxies in
our sample.
6.2.5. Potential Link between Burstiness and Stellar Clumps
Star-forming clumps observed in galaxies may present an
interesting connection to SFH burstiness explored in this work.
Giant, star-forming clumps are known to be prevalent in
galaxies with stellar masses as high as 109
Me and specific
SFRs up to 10−7
yr−1
. While they may be rare in the local
universe (Fisher et al. 2017; Mehta et al. 2021; Adams et al.
2022), these clumps are found in abundance at high redshifts
(0.5 < z < 2; e.g., Guo et al. 2015, 2018; Shibuya et al. 2016),
with the fraction of star-forming galaxies containing clumps
reaching ∼60% at z ∼ 2 (Guo et al. 2015). These clumps are
sites of active star formation and it is possible that these are
linked to the burstiness in galaxy SFHs. Clumps have been
confirmed to have a higher SFR compared to their galactic
surroundings and could be the physical manifestations of the
bursts in galaxy SFHs.
Guo et al. (2015) find a significantly higher clumpy fraction
for low-mass galaxies (<109.8
Me) relative to their high-mass
counterparts. This has similarities to our result, where low-mass
galaxies are found to be more bursty, in general. The clumps
found in the outskirts of galaxies are found to have higher SFR
than those in the inner regions (Guo et al. 2018), which is also
consistent with our result, which finds an increasing burstiness
at larger galactocentric distances. While no explicit link has
been established between burstiness in galaxy SFH and clumps
thus far, it remains an interesting avenue to explore, both from
an observational as well as a theoretical perspective.
7. Conclusions
The combination of UVCANDELS WFC3/UVIS imaging
along with 3D-HST+AGHAST WFC3/IR slitless grism
spectroscopy has provided a unique opportunity to study the
resolved rest-frame UV and Hα emission from the same
galaxies at z ∼ 1. Both rest-frame UV and Hα are excellent
tracers of ongoing star formation activity in galaxies; however,
since they are sensitive to different timescales, a comparison of
the two enables us to infer the burstiness in the SFH.
In this work, we select a sample of 979 star-forming galaxies
over 0.7 < z < 1.5 to compare their resolved rest-frame UV and
Hα emission and investigate the burstiness of their SFHs as a
function of their galaxy structural parameters. We start by
homogenizing the rest-UV and Hα maps by aligning,
calibrating, and PSF matching, as well as correcting them for
dust. We then compute the UV-to-Hα ratio (ζ), which serves as
13
The Astrophysical Journal, 952:133 (17pp), 2023 August 1 Mehta et al.
14. a means to parameterize the “burstiness” in star formation. We
perform a stacking analysis to generate averaged rest-UV and
Hα light profiles and compute the evolution of ζ as a function
of the galactocentric distance as well as the surface brightness.
We perform extensive tests to ensure that the observed trends
are not driven by any of the assumptions involved in our
analysis, including dust corrections, IMF assumptions, and
[N II] correction applied to the blended Hα +[N II] in grism
observations.
Our main results are as follows:
1. We find the ζ averaged over the full galaxy sample to
increase toward larger galactocentric distances. An
increasing contribution from a bursty SFH toward the
outskirts is one plausible explanation for this trend. A
smoothly declining SFH with an increasingly sharper
decline toward the galactic outskirts can also explain the
observed trend.
2. We further split our sample into three bins according to
the galaxy stellar mass. We find the low- and inter-
mediate-mass samples exhibit ζ that is roughly constant
and indicative of little to no evolution in the galaxy SFH
as a function of radius. The higher-than-equilibrium value
of ζ is also indicative of bursty SFHs, which is consistent
with existing studies that suggest galaxies below
∼109
Me to be dominated by bursty star formation.
3. On the other hand, we find a significant radial evolution
in ζ for the high-mass sample, consistent with secular star
formation in the interior regions (R/Reff < 1), but rapidly
deviating from the equilibrium value toward the outskirts
(R/Reff > 1.5), potentially indicative of the outskirts
having a bursty star-forming component. This suggests
that while the integrated photometry of high-mass
galaxies does not indicate the presence of burstiness in
their SFHs, the star formation in their outskirts could still
have a significant bursty component that is likely being
overshadowed by the brighter cores that dominate the
flux contribution.
4. We also compute ζ as a function of the surface brightness
(in F140W), which serves as a proxy for galaxy stellar
mass surface density. We recover similar trends as in the
case with radius, albeit they are more significant,
suggesting that the local stellar mass surface density is
the more fundamental factor in regulating the star
formation activity, rather than just distance from the
center.
5. Interestingly, we find that regardless of the galaxy stellar
mass, regions with a mass surface density below
∼107.5
Me kpc−2
have a significantly bursty star
formation, potentially indicating that local star formation
is independent of global galaxy properties at the smallest
scales.
6. Last, we find that the evolution of ζ as a function of the
radius and surface brightness is most strongly driven by
the star-forming population at z < 1.1. The galaxies at
z > 1.1 appear to have flat ζ trends, indicating that their
SFHs, while still bursty, do not evolve with radius or
surface brightness.
The trends in ζ presented in this work will serve as key
observational constraints for theoretical galaxy models. A
comparison between these observational results and current
state-of-the-art hydrodynamical simulations that include the
effects of stellar feedback and have the resolution to capture
star formation on <10 Myr timescales is bound to yield crucial
insights into our understanding of how star formation proceeds
in galaxies. We leave this analysis for a future publication.
Future observations with JWST will make Hα accessible at
higher redshifts (z > 1.5) and will also push to fainter, lower-
mass galaxies than those considered in this work. Combining
these with HST/UVIS imaging will allow us to push this
analysis to earlier galaxies and uncover the properties of their
star formation processes at the peak of cosmic noon.
Acknowledgments
We would like to acknowledge Dr. Gabriel Brammer for
their critical contribution in the GRIZLI reduction of 3D-HST
grism spectroscopy data as part of the Complete Hubble
Archive for Galaxy Evolution (CHArGE) initiative. The
analysis presented in this paper is based on observations with
the NASA/ESA Hubble Space Telescope obtained at the Space
Telescope Science Institute, which is operated by the Associa-
tion of Universities for Research in Astronomy, Incorporated,
under NASA contract NAS5-26555. Support for Program
Number HST-GO-15647 was provided through a grant from
the STScI under NASA contract NAS5-26555. V.M., H.T., and
X.W. acknowledge work carried out, in part, by IPAC at the
California Institute of Technology, as was sponsored by the
National Aeronautics and Space Administration.
Facilities: HST (WFC3), HST (ACS).
Software: NumPy, SciPy, AstroPy, Matplotlib, GRIZLI
(Brammer & Matharu 2021), SWARP (Bertin et al. 2002),
PYPHER (Boucaud et al. 2016), DENSE BASIS (Iyer
et al. 2019).
Appendix
Individual Light Profiles for Subsamples
Figure 9 shows the stacked light profiles as a function of
galactocentric radius (top row) as well as surface brightness
(bottom row) for each of the subsamples binned in galaxy
stellar mass. For each case, in addition to our fiducial result, we
also show the profiles computed assuming the SMC dust law
(shown as the dashed curves) and the Buat et al. (2018) dust
law (shown as the dotted curves). Figure 10 shows the results
for our two test dust correction scenarios: (i) assuming a single
value of dust per galaxy (shown as the dashed curves); and (ii)
stacking only the pixels with a reliable per-pixel dust estimate
(shown as the dotted curves).
14
The Astrophysical Journal, 952:133 (17pp), 2023 August 1 Mehta et al.
15. Figure 9. Similar to the left panel of Figure 5, but shown for each subsample binned in galaxy stellar mass and as a function of galactocentric distance (top row) as
well as surface brightness (bottom row).
15
The Astrophysical Journal, 952:133 (17pp), 2023 August 1 Mehta et al.
16. ORCID iDs
Vihang Mehta https:/
/orcid.org/0000-0001-7166-6035
Harry I. Teplitz https:/
/orcid.org/0000-0002-7064-5424
Claudia Scarlata https:/
/orcid.org/0000-0002-9136-8876
Xin Wang https:/
/orcid.org/0000-0002-9373-3865
Anahita Alavi https:/
/orcid.org/0000-0002-8630-6435
James Colbert https:/
/orcid.org/0000-0001-6482-3020
Marc Rafelski https:/
/orcid.org/0000-0002-9946-4731
Norman Grogin https:/
/orcid.org/0000-0001-9440-8872
Anton Koekemoer https:/
/orcid.org/0000-0002-6610-2048
Laura Prichard https:/
/orcid.org/0000-0002-0604-654X
Rogier Windhorst https:/
/orcid.org/0000-0001-8156-6281
Christopher J. Conselice https:/
/orcid.org/0000-0003-
1949-7638
Y. Sophia Dai (戴昱) https:/
/orcid.org/0000-0002-
7928-416X
Jonathan P. Gardner https:/
/orcid.org/0000-0003-
2098-9568
Eric Gawiser https:/
/orcid.org/0000-0003-1530-8713
Yicheng Guo https:/
/orcid.org/0000-0003-2775-2002
Nimish Hathi https:/
/orcid.org/0000-0001-6145-5090
Pablo Arrabal Haro https:/
/orcid.org/0000-0002-7959-8783
Matthew Hayes https:/
/orcid.org/0000-0001-8587-218X
Kartheik G. Iyer https:/
/orcid.org/0000-0001-9298-3523
Rolf A. Jansen https:/
/orcid.org/0000-0003-1268-5230
Zhiyuan Ji https:/
/orcid.org/0000-0001-7673-2257
Peter Kurczynski https:/
/orcid.org/0000-0002-8816-5146
Maxwell Kuschel https:/
/orcid.org/0000-0002-7830-363X
Ray A. Lucas https:/
/orcid.org/0000-0003-1581-7825
Kameswara Mantha https:/
/orcid.org/0000-0002-
6016-300X
Robert W. O’Connell https:/
/orcid.org/0000-0002-8190-7573
Swara Ravindranath https:/
/orcid.org/0000-0002-
5269-6527
Brant E. Robertson https:/
/orcid.org/0000-0002-4271-0364
Michael Rutkowski https:/
/orcid.org/0000-0001-7016-5220
Brian Siana https:/
/orcid.org/0000-0002-4935-9511
L. Y. Aaron Yung https:/
/orcid.org/0000-0003-3466-035X
Figure 10. Similar to the right panel of Figure 5, but shown for each subsample binned in galaxy stellar mass and as a function of galactocentric distance (top row) as
well as surface brightness (bottom row).
16
The Astrophysical Journal, 952:133 (17pp), 2023 August 1 Mehta et al.
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