1. Chandra observations of the galaxy NGC 3351 revealed X-ray emission from its circumnuclear star-forming ring that is composed of numerous point-like sources embedded in diffuse hot gas.
2. The morphology of the X-ray emission is similar to but not identical with UV and H-alpha hot spots in the ring, which can be understood if star formation occurs through intermittent starbursts around the ring with different emissions tracing later evolutionary stages.
3. X-ray emission also extends beyond the ring, which is interpreted as outflowing gas from the ring into the disk and halo of NGC 3351, providing evidence for confined outflow near the plane but less restricted outflow perpendicular to
The physical conditions_in_a_pre_super_star_cluster_molecular_cloud_in_the_an...Sérgio Sacani
Artigo descreve estudo feitos pelos astrônomos utilizando o ALMA para descobrir um proto-aglomerado globular de estrelas gigantes se formando no interior das galáxias Antenas, o famoso par de galáxias em interação. É a primeira vez que os astrônomos conseguem observar um objeto desse tipo nos seus estágios iniciais de vida e com o ambiente ao redor inalterado.
The physical conditions_in_a_pre_super_star_cluster_molecular_cloud_in_the_an...Sérgio Sacani
Artigo descreve estudo feitos pelos astrônomos utilizando o ALMA para descobrir um proto-aglomerado globular de estrelas gigantes se formando no interior das galáxias Antenas, o famoso par de galáxias em interação. É a primeira vez que os astrônomos conseguem observar um objeto desse tipo nos seus estágios iniciais de vida e com o ambiente ao redor inalterado.
Evidence for reflected_lightfrom_the_most_eccentric_exoplanet_knownSérgio Sacani
Planets in highly eccentric orbits form a class of objects not seen within our Solar System. The most extreme case known amongst these objects is the planet orbiting HD 20782, with an orbital period of 597 days and an eccentricity of 0.96. Here we present new data and analysis for this system as part of the Transit Ephemeris Refinement and Monitoring Survey (TERMS). We obtained CHIRON spectra to perform an independent estimation of the fundamental stellar parameters. New radial velocities from AAT and PARAS observations during periastron passage greatly improve our knowledge of the eccentric nature of the orbit. The combined analysis of our Keplerian orbital and Hipparcos astrometry show that the inclination of the planetary orbit is > 1.22◦, ruling out stellar masses for the companion. Our long-term robotic photometry show that the star is extremely stable over long timescales. Photometric monitoring of the star during predicted transit and periastron times using MOST rule out a transit of the planet and reveal evidence of phase variations during periastron. These possible photometric phase variations may be caused by reflected light from the planet’s atmosphere and the dramatic change in star–planet separation surrounding the periastron passage.
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.
The puzzling source_in_ngc6388_a_possible_planetary_tidal_disruption_eventSérgio Sacani
Artigo descreve a descoberta da destruição de um planeta ao passar próximo a uma estrela do tipo anã branca presente dentro do aglomerado globular de estrelas NGC 6388. Para isso os astrônomos utilizaram um arsenal de telescópios.
We discovered two transient events in the Kepler eld with light curves that strongly suggest they
are type II-P supernovae. Using the fast cadence of the Kepler observations we precisely estimate
the rise time to maximum for KSN2011a and KSN2011d as 10.50:4 and 13.30:4 rest-frame days
respectively. Based on ts to idealized analytic models, we nd the progenitor radius of KSN2011a
(28020 R) to be signicantly smaller than that for KSN2011d (49020 R) but both have similar
explosion energies of 2.00:3 1051 erg.
The rising light curve of KSN2011d is an excellent match to that predicted by simple models of
exploding red supergiants (RSG). However, the early rise of KSN2011a is faster than the models
predict possibly due to the supernova shockwave moving into pre-existing wind or mass-loss from the
RSG. A mass loss rate of 10 4 M yr 1 from the RSG can explain the fast rise without impacting
the optical
ux at maximum light or the shape of the post-maximum light curve.
No shock breakout emission is seen in KSN2011a, but this is likely due to the circumstellar inter-
action suspected in the fast rising light curve. The early light curve of KSN2011d does show excess
emission consistent with model predictions of a shock breakout. This is the rst optical detection of
a shock breakout from a type II-P supernova.
Periodic mass extinctions_and_the_planet_x_model_reconsideredSérgio Sacani
The 27 Myr periodicity in the fossil extinction record has been con-
firmed in modern data bases dating back 500 Myr, which is twice the time
interval of the original analysis from thirty years ago. The surprising regularity
of this period has been used to reject the Nemesis model. A second
model based on the sun’s vertical galactic oscillations has been challenged
on the basis of an inconsistency in period and phasing. The third astronomical
model originally proposed to explain the periodicity is the Planet
X model in which the period is associated with the perihelion precession
of the inclined orbit of a trans-Neptunian planet. Recently, and unrelated
to mass extinctions, a trans-Neptunian super-Earth planet has been proposed
to explain the observation that the inner Oort cloud objects Sedna
and 2012VP113 have perihelia that lie near the ecliptic plane. In this
Letter we reconsider the Planet X model in light of the confluence of the
modern palaeontological and outer solar system dynamical evidence.
Key Words: astrobiology - planets and satellites - Kuiper belt:
general - comets: general
A luminous X-ray outburst from an intermediatemass black hole in an off-centr...Sérgio Sacani
A unique signature for the presence of massive black holes in
very dense stellar regions is occasional giant-amplitude outbursts
of multi-wavelength radiation from tidal disruption and
subsequent accretion of stars that make a close approach to
the black holes1
. Previous strong tidal disruption event (TDE)
candidates were all associated with the centres of largely
isolated galaxies2–6. Here, we report the discovery of a luminous
X-ray outburst from a massive star cluster at a projected
distance of 12.5 kpc from the centre of a large lenticular galaxy.
The luminosity peaked at ~1043 erg s−1
and decayed systematically
over 10 years, approximately following a trend
that supports the identification of the event as a TDE. The
X-ray spectra were all very soft, with emission confined to be
≲3.0 keV, and could be described with a standard thermal disk.
The disk cooled significantly as the luminosity decreased—a
key thermal-state signature often observed in accreting stellar-mass
black holes. This thermal-state signature, coupled
with very high luminosities, ultrasoft X-ray spectra and the
characteristic power-law evolution of the light curve, provides
strong evidence that the source contains an intermediatemass
black hole with a mass tens of thousand times that of
the solar mass. This event demonstrates that one of the most
effective means of detecting intermediate-mass black holes is
through X-ray flares from TDEs in star clusters.
Os astrônomos descobriram um processo único sobre como as maiores galáxias elípticas do universo continuam gerando estrelas muito tempo depois do anos de pico de nascimentos estelares. A alta resolução e a sensibilidade à radiação ultravioleta do Hubble, permitiu aos astrônomos observarem nós brilhantes de estrelas azuis, quentes, se formando juntamente com jatos de buracos negros ativos encontrados nos centros das gigantescas galáxias elípticas.
Combinando dados do Huubble com observações feitas por um conjunto de telescópios baseados tanto em Terra como no espaço, duas equipes independentes descobriram que os jatos dos buracos negros, e as estrelas recém-nascidas são todos partes de um ciclo auto-regulado. Jatos de alta energia atirados do buraco negro aquecem um halo de gás circulante, controlando a taxa com a qual o gás esfria e cai na galáxia.
“Pense no gás ao redor da galáxia como uma atmosfera”, explicou o líder do primeiro estudo, Megan Donahue, da Universidade Estadual do Michigan. “Essa atmosfera pode conter material em diferentes estados, do mesmo modo que a nossa atmosfera tem gás, nuvens e chuva. O que nós estamos vendo é um processo parecido com uma tempestade. À medida que os jatos impulsionam o gás para fora do centro da galáxia, parte do gás esfria e precipita em aglomerados frios que caem de volta para o centro da galáxia como gotas de chuvas”.
“As gotas de chuva eventualmente esfriam o suficiente para tornar-se nuvens de formação de estrelas de gás frio molecular, e a capacidade de observar no ultravioleta distante do Hubble, nos permitiu observar diretamente esses chuviscos de formação de estrelas”, explicou o líder do segundo estudo, Grant Tremblay, da Universidade de Yale. “Nós sabemos que esses chuviscos estão linkados com os jatos, pois eles foram encontrados em filamentos que se dobram ao redor dos jatos, ou abraçam as bordas de bolhas gigantes que os jatos inflaram”, disse Tremblay. “E eles terminam fazendo um redemoinho de gás de formação de estrelas ao redor do buraco negro central”.
The characterization of_the_gamma_ray_signal_from_the_central_milk_way_a_comp...Sérgio Sacani
Past studies have identified a spatially extended excess of ∼1-3 GeV gamma rays from the region
surrounding the Galactic Center, consistent with the emission expected from annihilating dark
matter. We revisit and scrutinize this signal with the intention of further constraining its characteristics
and origin. By applying cuts to the Fermi event parameter CTBCORE, we suppress the tails
of the point spread function and generate high resolution gamma-ray maps, enabling us to more
easily separate the various gamma-ray components. Within these maps, we find the GeV excess
to be robust and highly statistically significant, with a spectrum, angular distribution, and overall
normalization that is in good agreement with that predicted by simple annihilating dark matter
models. For example, the signal is very well fit by a 36-51 GeV dark matter particle annihilating to
b
¯b with an annihilation cross section of σv = (1−3)×10−26 cm3
/s (normalized to a local dark matter
density of 0.4 GeV/cm3
). Furthermore, we confirm that the angular distribution of the excess is
approximately spherically symmetric and centered around the dynamical center of the Milky Way
(within ∼0.05◦
of Sgr A∗
), showing no sign of elongation along the Galactic Plane. The signal is
observed to extend to at least ' 10◦
from the Galactic Center, disfavoring the possibility that this
emission originates from millisecond pulsars.
Evidence for reflected_lightfrom_the_most_eccentric_exoplanet_knownSérgio Sacani
Planets in highly eccentric orbits form a class of objects not seen within our Solar System. The most extreme case known amongst these objects is the planet orbiting HD 20782, with an orbital period of 597 days and an eccentricity of 0.96. Here we present new data and analysis for this system as part of the Transit Ephemeris Refinement and Monitoring Survey (TERMS). We obtained CHIRON spectra to perform an independent estimation of the fundamental stellar parameters. New radial velocities from AAT and PARAS observations during periastron passage greatly improve our knowledge of the eccentric nature of the orbit. The combined analysis of our Keplerian orbital and Hipparcos astrometry show that the inclination of the planetary orbit is > 1.22◦, ruling out stellar masses for the companion. Our long-term robotic photometry show that the star is extremely stable over long timescales. Photometric monitoring of the star during predicted transit and periastron times using MOST rule out a transit of the planet and reveal evidence of phase variations during periastron. These possible photometric phase variations may be caused by reflected light from the planet’s atmosphere and the dramatic change in star–planet separation surrounding the periastron passage.
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.
The puzzling source_in_ngc6388_a_possible_planetary_tidal_disruption_eventSérgio Sacani
Artigo descreve a descoberta da destruição de um planeta ao passar próximo a uma estrela do tipo anã branca presente dentro do aglomerado globular de estrelas NGC 6388. Para isso os astrônomos utilizaram um arsenal de telescópios.
We discovered two transient events in the Kepler eld with light curves that strongly suggest they
are type II-P supernovae. Using the fast cadence of the Kepler observations we precisely estimate
the rise time to maximum for KSN2011a and KSN2011d as 10.50:4 and 13.30:4 rest-frame days
respectively. Based on ts to idealized analytic models, we nd the progenitor radius of KSN2011a
(28020 R) to be signicantly smaller than that for KSN2011d (49020 R) but both have similar
explosion energies of 2.00:3 1051 erg.
The rising light curve of KSN2011d is an excellent match to that predicted by simple models of
exploding red supergiants (RSG). However, the early rise of KSN2011a is faster than the models
predict possibly due to the supernova shockwave moving into pre-existing wind or mass-loss from the
RSG. A mass loss rate of 10 4 M yr 1 from the RSG can explain the fast rise without impacting
the optical
ux at maximum light or the shape of the post-maximum light curve.
No shock breakout emission is seen in KSN2011a, but this is likely due to the circumstellar inter-
action suspected in the fast rising light curve. The early light curve of KSN2011d does show excess
emission consistent with model predictions of a shock breakout. This is the rst optical detection of
a shock breakout from a type II-P supernova.
Periodic mass extinctions_and_the_planet_x_model_reconsideredSérgio Sacani
The 27 Myr periodicity in the fossil extinction record has been con-
firmed in modern data bases dating back 500 Myr, which is twice the time
interval of the original analysis from thirty years ago. The surprising regularity
of this period has been used to reject the Nemesis model. A second
model based on the sun’s vertical galactic oscillations has been challenged
on the basis of an inconsistency in period and phasing. The third astronomical
model originally proposed to explain the periodicity is the Planet
X model in which the period is associated with the perihelion precession
of the inclined orbit of a trans-Neptunian planet. Recently, and unrelated
to mass extinctions, a trans-Neptunian super-Earth planet has been proposed
to explain the observation that the inner Oort cloud objects Sedna
and 2012VP113 have perihelia that lie near the ecliptic plane. In this
Letter we reconsider the Planet X model in light of the confluence of the
modern palaeontological and outer solar system dynamical evidence.
Key Words: astrobiology - planets and satellites - Kuiper belt:
general - comets: general
A luminous X-ray outburst from an intermediatemass black hole in an off-centr...Sérgio Sacani
A unique signature for the presence of massive black holes in
very dense stellar regions is occasional giant-amplitude outbursts
of multi-wavelength radiation from tidal disruption and
subsequent accretion of stars that make a close approach to
the black holes1
. Previous strong tidal disruption event (TDE)
candidates were all associated with the centres of largely
isolated galaxies2–6. Here, we report the discovery of a luminous
X-ray outburst from a massive star cluster at a projected
distance of 12.5 kpc from the centre of a large lenticular galaxy.
The luminosity peaked at ~1043 erg s−1
and decayed systematically
over 10 years, approximately following a trend
that supports the identification of the event as a TDE. The
X-ray spectra were all very soft, with emission confined to be
≲3.0 keV, and could be described with a standard thermal disk.
The disk cooled significantly as the luminosity decreased—a
key thermal-state signature often observed in accreting stellar-mass
black holes. This thermal-state signature, coupled
with very high luminosities, ultrasoft X-ray spectra and the
characteristic power-law evolution of the light curve, provides
strong evidence that the source contains an intermediatemass
black hole with a mass tens of thousand times that of
the solar mass. This event demonstrates that one of the most
effective means of detecting intermediate-mass black holes is
through X-ray flares from TDEs in star clusters.
Os astrônomos descobriram um processo único sobre como as maiores galáxias elípticas do universo continuam gerando estrelas muito tempo depois do anos de pico de nascimentos estelares. A alta resolução e a sensibilidade à radiação ultravioleta do Hubble, permitiu aos astrônomos observarem nós brilhantes de estrelas azuis, quentes, se formando juntamente com jatos de buracos negros ativos encontrados nos centros das gigantescas galáxias elípticas.
Combinando dados do Huubble com observações feitas por um conjunto de telescópios baseados tanto em Terra como no espaço, duas equipes independentes descobriram que os jatos dos buracos negros, e as estrelas recém-nascidas são todos partes de um ciclo auto-regulado. Jatos de alta energia atirados do buraco negro aquecem um halo de gás circulante, controlando a taxa com a qual o gás esfria e cai na galáxia.
“Pense no gás ao redor da galáxia como uma atmosfera”, explicou o líder do primeiro estudo, Megan Donahue, da Universidade Estadual do Michigan. “Essa atmosfera pode conter material em diferentes estados, do mesmo modo que a nossa atmosfera tem gás, nuvens e chuva. O que nós estamos vendo é um processo parecido com uma tempestade. À medida que os jatos impulsionam o gás para fora do centro da galáxia, parte do gás esfria e precipita em aglomerados frios que caem de volta para o centro da galáxia como gotas de chuvas”.
“As gotas de chuva eventualmente esfriam o suficiente para tornar-se nuvens de formação de estrelas de gás frio molecular, e a capacidade de observar no ultravioleta distante do Hubble, nos permitiu observar diretamente esses chuviscos de formação de estrelas”, explicou o líder do segundo estudo, Grant Tremblay, da Universidade de Yale. “Nós sabemos que esses chuviscos estão linkados com os jatos, pois eles foram encontrados em filamentos que se dobram ao redor dos jatos, ou abraçam as bordas de bolhas gigantes que os jatos inflaram”, disse Tremblay. “E eles terminam fazendo um redemoinho de gás de formação de estrelas ao redor do buraco negro central”.
The characterization of_the_gamma_ray_signal_from_the_central_milk_way_a_comp...Sérgio Sacani
Past studies have identified a spatially extended excess of ∼1-3 GeV gamma rays from the region
surrounding the Galactic Center, consistent with the emission expected from annihilating dark
matter. We revisit and scrutinize this signal with the intention of further constraining its characteristics
and origin. By applying cuts to the Fermi event parameter CTBCORE, we suppress the tails
of the point spread function and generate high resolution gamma-ray maps, enabling us to more
easily separate the various gamma-ray components. Within these maps, we find the GeV excess
to be robust and highly statistically significant, with a spectrum, angular distribution, and overall
normalization that is in good agreement with that predicted by simple annihilating dark matter
models. For example, the signal is very well fit by a 36-51 GeV dark matter particle annihilating to
b
¯b with an annihilation cross section of σv = (1−3)×10−26 cm3
/s (normalized to a local dark matter
density of 0.4 GeV/cm3
). Furthermore, we confirm that the angular distribution of the excess is
approximately spherically symmetric and centered around the dynamical center of the Milky Way
(within ∼0.05◦
of Sgr A∗
), showing no sign of elongation along the Galactic Plane. The signal is
observed to extend to at least ' 10◦
from the Galactic Center, disfavoring the possibility that this
emission originates from millisecond pulsars.
Solving the Multimessenger Puzzle of the AGN-starburst Composite Galaxy NGC 1068Sérgio Sacani
Multiwavelength observations indicate that some starburst galaxies show a dominant nonthermal contribution from
their central region. These active galactic nuclei (AGN)-starburst composites are of special interest, as both
phenomena on their own are potential sources of highly energetic cosmic rays and associated γ-ray and neutrino
emission. In this work, a homogeneous, steady-state two-zone multimessenger model of the nonthermal emission
from the AGN corona as well as the circumnuclear starburst region is developed and subsequently applied to the
case of NGC 1068, which has recently shown some first indications of high-energy neutrino emission. Here, we
show that the entire spectrum of multimessenger data—from radio to γ-rays including the neutrino constraint—can
be described very well if both, starburst and AGN corona, are taken into account. Using only a single emission
region is not sufficient.
A deep dive: Chandra observations of the NGC 4839 group falling into the Coma...Sérgio Sacani
Cosmological simulations of structure formation predict that galaxy clusters continue to grow and evolve through ongoing
mergers with group-scale systems. During these merging events, the ram pressure applied by the intracluster medium acts to strip
the gas from the infalling groups, forming large tails of stripped gas, which eventually become part of the main cluster. In this
work, we present a detailed analysis of our new deep Chandra observations of the NGC 4839 group falling into the nearby Coma
cluster, providing a unique opportunity to explore the way galaxy clusters in the local universe continue to grow. Our analysis
reveals a cold front feature at the leading head of the group, preceded by a bow shock of hot gas in front with a Mach number
of ∼ 1.5. The power spectrum of surface brightness fluctuations in the tail shows that the slope gets less steep as the distance
from the leading head increases, changing from −2.35+0.07
−0.06 at the inner part of the tail to −1.37+0.09
−0.07 at the outermost part of
the tail. These values are shallower than the slope of the Kolmogorov 2D power spectrum, indicating that thermal conduction is
being suppressed throughout the tail, enabling long-lived small-scale turbulence, which would typically be washed out if thermal
conduction was not inhibited. The characteristic amplitude of surface brightness fluctuations in the tail suggests a mild level of
turbulence with a Mach number in the range of 0.1–0.5, agreeing with that found for the infalling group in Abell 2142.
The xmm newton-view_of_the_central_degrees_of_the_milk_waySérgio Sacani
Novas imagens do Observatório de Raios-X XMM-Newton da ESA revelaram alguns dos processos mais intensos que acontecem no coração da nossa Via Láctea.
As fontes brilhantes e pontuais que se destacam por toda imagem indicam os sistemas estelares binários onde uma das estrelas atingiu o final de sua vida, desenvolvendo para um objeto compacto e denso – uma estrela de nêutrons ou um buraco negro.
A região central da Via Láctea também contém jovens estrelas e aglomerados estelares e algumas dessas fontes são visíveis como pontos brancos e vermelhos brilhando na imagem, que se espalha por 1000 anos-luz.
A maior parte da ação ocorre no centro, onde nuvens difusas de gás estão sendo cavadas por ventos poderosos soprados por estrelas jovens, bem como por supernovas.
Similar to Chandra observation star_formation_m95 (20)
The binding of cosmological structures by massless topological defectsSérgio Sacani
Assuming spherical symmetry and weak field, it is shown that if one solves the Poisson equation or the Einstein field
equations sourced by a topological defect, i.e. a singularity of a very specific form, the result is a localized gravitational
field capable of driving flat rotation (i.e. Keplerian circular orbits at a constant speed for all radii) of test masses on a thin
spherical shell without any underlying mass. Moreover, a large-scale structure which exploits this solution by assembling
concentrically a number of such topological defects can establish a flat stellar or galactic rotation curve, and can also deflect
light in the same manner as an equipotential (isothermal) sphere. Thus, the need for dark matter or modified gravity theory is
mitigated, at least in part.
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.
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.
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.
Epistemic Interaction - tuning interfaces to provide information for AI supportAlan Dix
Paper presented at SYNERGY workshop at AVI 2024, Genoa, Italy. 3rd June 2024
https://alandix.com/academic/papers/synergy2024-epistemic/
As machine learning integrates deeper into human-computer interactions, the concept of epistemic interaction emerges, aiming to refine these interactions to enhance system adaptability. This approach encourages minor, intentional adjustments in user behaviour to enrich the data available for system learning. This paper introduces epistemic interaction within the context of human-system communication, illustrating how deliberate interaction design can improve system understanding and adaptation. Through concrete examples, we demonstrate the potential of epistemic interaction to significantly advance human-computer interaction by leveraging intuitive human communication strategies to inform system design and functionality, offering a novel pathway for enriching user-system engagements.
DevOps and Testing slides at DASA ConnectKari Kakkonen
My and Rik Marselis slides at 30.5.2024 DASA Connect conference. We discuss about what is testing, then what is agile testing and finally what is Testing in DevOps. Finally we had lovely workshop with the participants trying to find out different ways to think about quality and testing in different parts of the DevOps infinity loop.
Removing Uninteresting Bytes in Software FuzzingAftab Hussain
Imagine a world where software fuzzing, the process of mutating bytes in test seeds to uncover hidden and erroneous program behaviors, becomes faster and more effective. A lot depends on the initial seeds, which can significantly dictate the trajectory of a fuzzing campaign, particularly in terms of how long it takes to uncover interesting behaviour in your code. We introduce DIAR, a technique designed to speedup fuzzing campaigns by pinpointing and eliminating those uninteresting bytes in the seeds. Picture this: instead of wasting valuable resources on meaningless mutations in large, bloated seeds, DIAR removes the unnecessary bytes, streamlining the entire process.
In this work, we equipped AFL, a popular fuzzer, with DIAR and examined two critical Linux libraries -- Libxml's xmllint, a tool for parsing xml documents, and Binutil's readelf, an essential debugging and security analysis command-line tool used to display detailed information about ELF (Executable and Linkable Format). Our preliminary results show that AFL+DIAR does not only discover new paths more quickly but also achieves higher coverage overall. This work thus showcases how starting with lean and optimized seeds can lead to faster, more comprehensive fuzzing campaigns -- and DIAR helps you find such seeds.
- These are slides of the talk given at IEEE International Conference on Software Testing Verification and Validation Workshop, ICSTW 2022.
LF Energy Webinar: Electrical Grid Modelling and Simulation Through PowSyBl -...DanBrown980551
Do you want to learn how to model and simulate an electrical network from scratch in under an hour?
Then welcome to this PowSyBl workshop, hosted by Rte, the French Transmission System Operator (TSO)!
During the webinar, you will discover the PowSyBl ecosystem as well as handle and study an electrical network through an interactive Python notebook.
PowSyBl is an open source project hosted by LF Energy, which offers a comprehensive set of features for electrical grid modelling and simulation. Among other advanced features, PowSyBl provides:
- A fully editable and extendable library for grid component modelling;
- Visualization tools to display your network;
- Grid simulation tools, such as power flows, security analyses (with or without remedial actions) and sensitivity analyses;
The framework is mostly written in Java, with a Python binding so that Python developers can access PowSyBl functionalities as well.
What you will learn during the webinar:
- For beginners: discover PowSyBl's functionalities through a quick general presentation and the notebook, without needing any expert coding skills;
- For advanced developers: master the skills to efficiently apply PowSyBl functionalities to your real-world scenarios.
The Art of the Pitch: WordPress Relationships and SalesLaura Byrne
Clients don’t know what they don’t know. What web solutions are right for them? How does WordPress come into the picture? How do you make sure you understand scope and timeline? What do you do if sometime changes?
All these questions and more will be explored as we talk about matching clients’ needs with what your agency offers without pulling teeth or pulling your hair out. Practical tips, and strategies for successful relationship building that leads to closing the deal.
Alt. GDG Cloud Southlake #33: Boule & Rebala: Effective AppSec in SDLC using ...James Anderson
Effective Application Security in Software Delivery lifecycle using Deployment Firewall and DBOM
The modern software delivery process (or the CI/CD process) includes many tools, distributed teams, open-source code, and cloud platforms. Constant focus on speed to release software to market, along with the traditional slow and manual security checks has caused gaps in continuous security as an important piece in the software supply chain. Today organizations feel more susceptible to external and internal cyber threats due to the vast attack surface in their applications supply chain and the lack of end-to-end governance and risk management.
The software team must secure its software delivery process to avoid vulnerability and security breaches. This needs to be achieved with existing tool chains and without extensive rework of the delivery processes. This talk will present strategies and techniques for providing visibility into the true risk of the existing vulnerabilities, preventing the introduction of security issues in the software, resolving vulnerabilities in production environments quickly, and capturing the deployment bill of materials (DBOM).
Speakers:
Bob Boule
Robert Boule is a technology enthusiast with PASSION for technology and making things work along with a knack for helping others understand how things work. He comes with around 20 years of solution engineering experience in application security, software continuous delivery, and SaaS platforms. He is known for his dynamic presentations in CI/CD and application security integrated in software delivery lifecycle.
Gopinath Rebala
Gopinath Rebala is the CTO of OpsMx, where he has overall responsibility for the machine learning and data processing architectures for Secure Software Delivery. Gopi also has a strong connection with our customers, leading design and architecture for strategic implementations. Gopi is a frequent speaker and well-known leader in continuous delivery and integrating security into software delivery.
UiPath Test Automation using UiPath Test Suite series, part 6DianaGray10
Welcome to UiPath Test Automation using UiPath Test Suite series part 6. In this session, we will cover Test Automation with generative AI and Open AI.
UiPath Test Automation with generative AI and Open AI webinar offers an in-depth exploration of leveraging cutting-edge technologies for test automation within the UiPath platform. Attendees will delve into the integration of generative AI, a test automation solution, with Open AI advanced natural language processing capabilities.
Throughout the session, participants will discover how this synergy empowers testers to automate repetitive tasks, enhance testing accuracy, and expedite the software testing life cycle. Topics covered include the seamless integration process, practical use cases, and the benefits of harnessing AI-driven automation for UiPath testing initiatives. By attending this webinar, testers, and automation professionals can gain valuable insights into harnessing the power of AI to optimize their test automation workflows within the UiPath ecosystem, ultimately driving efficiency and quality in software development processes.
What will you get from this session?
1. Insights into integrating generative AI.
2. Understanding how this integration enhances test automation within the UiPath platform
3. Practical demonstrations
4. Exploration of real-world use cases illustrating the benefits of AI-driven test automation for UiPath
Topics covered:
What is generative AI
Test Automation with generative AI and Open AI.
UiPath integration with generative AI
Speaker:
Deepak Rai, Automation Practice Lead, Boundaryless Group and UiPath MVP
Goodbye Windows 11: Make Way for Nitrux Linux 3.5.0!SOFTTECHHUB
As the digital landscape continually evolves, operating systems play a critical role in shaping user experiences and productivity. The launch of Nitrux Linux 3.5.0 marks a significant milestone, offering a robust alternative to traditional systems such as Windows 11. This article delves into the essence of Nitrux Linux 3.5.0, exploring its unique features, advantages, and how it stands as a compelling choice for both casual users and tech enthusiasts.
Sudheer Mechineni, Head of Application Frameworks, Standard Chartered Bank
Discover how Standard Chartered Bank harnessed the power of Neo4j to transform complex data access challenges into a dynamic, scalable graph database solution. This keynote will cover their journey from initial adoption to deploying a fully automated, enterprise-grade causal cluster, highlighting key strategies for modelling organisational changes and ensuring robust disaster recovery. Learn how these innovations have not only enhanced Standard Chartered Bank’s data infrastructure but also positioned them as pioneers in the banking sector’s adoption of graph technology.
Maruthi Prithivirajan, Head of ASEAN & IN Solution Architecture, Neo4j
Get an inside look at the latest Neo4j innovations that enable relationship-driven intelligence at scale. Learn more about the newest cloud integrations and product enhancements that make Neo4j an essential choice for developers building apps with interconnected data and generative AI.
UiPath Test Automation using UiPath Test Suite series, part 5DianaGray10
Welcome to UiPath Test Automation using UiPath Test Suite series part 5. In this session, we will cover CI/CD with devops.
Topics covered:
CI/CD with in UiPath
End-to-end overview of CI/CD pipeline with Azure devops
Speaker:
Lyndsey Byblow, Test Suite Sales Engineer @ UiPath, Inc.
Encryption in Microsoft 365 - ExpertsLive Netherlands 2024Albert Hoitingh
In this session I delve into the encryption technology used in Microsoft 365 and Microsoft Purview. Including the concepts of Customer Key and Double Key Encryption.
Communications Mining Series - Zero to Hero - Session 1DianaGray10
This session provides introduction to UiPath Communication Mining, importance and platform overview. You will acquire a good understand of the phases in Communication Mining as we go over the platform with you. Topics covered:
• Communication Mining Overview
• Why is it important?
• How can it help today’s business and the benefits
• Phases in Communication Mining
• Demo on Platform overview
• Q/A
In the rapidly evolving landscape of technologies, XML continues to play a vital role in structuring, storing, and transporting data across diverse systems. The recent advancements in artificial intelligence (AI) present new methodologies for enhancing XML development workflows, introducing efficiency, automation, and intelligent capabilities. This presentation will outline the scope and perspective of utilizing AI in XML development. The potential benefits and the possible pitfalls will be highlighted, providing a balanced view of the subject.
We will explore the capabilities of AI in understanding XML markup languages and autonomously creating structured XML content. Additionally, we will examine the capacity of AI to enrich plain text with appropriate XML markup. Practical examples and methodological guidelines will be provided to elucidate how AI can be effectively prompted to interpret and generate accurate XML markup.
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1. The Astrophysical Journal, 647:1030–1039, 2006 August 20
# 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A.
CHANDRA OBSERVATIONS OF CIRCUMNUCLEAR STAR FORMATION IN NGC 3351
Douglas A. Swartz,1 Mihoko Yukita,2 Allyn F. Tennant,3 Roberto Soria,4 and Kajal K. Ghosh1
Received 2005 December 30; accepted 2006 March 24
ABSTRACT
The nearby SB(r)b galaxy NGC 3351 (M95) displays an $600 pc diameter star-forming circumnuclear ring fueled
by gas accreted through a stellar bar. The X-ray emission from this region is composed of numerous pointlike sources
embedded in hot (kT ¼ 0:5 Æ 0:1 keV ), nonuniform diffuse gas. Most of the pointlike sources are likely themselves
concentrated regions of hot gas. The morphology of the X-ray emission is similar to, but not identical with, UV and
H hot spots in the circumnuclear ring. The emission morphology can be understood if star formation is occurring
through a series of instantaneous starbursts at various locations around the ring and the UV, H, and X-ray emission
trace successively later stages in the evolution of the underlying stellar populations. X-ray emission extends at least
500 pc beyond the circumnuclear ring. This emission is interpreted as the result of gas outflow from the star-forming
ring into the disk and halo of NGC 3351. There is evidence that the outflow is confined by cold ambient gas near the
plane of the galaxy but that flow out of the plane is not restricted. The nucleus of the galaxy lacks both large amounts
of hot gas and any pointlike source above an estimated X-ray luminosity of 1037 ergs sÀ1 in the 0.5–8.0 keV band.
This luminosity is orders of magnitude too low to account for the observed nuclear H luminosity, 3:5 ; 1038 ergs sÀ1,
by X-ray photoionization.
Subject headingg: galaxies: individual ( NGC 3351) — galaxies: starburst — galaxies: structure —
s
X-rays: galaxies
1. INTRODUCTION We have obtained Chandra X-ray images of NGC 3351 in
order to trace the effects of the circumnuclear ring, bar, and other
Central bars are a common feature of disk galaxies, occurring
in at least half of the population. In early-type spirals, the asym- structures on observed X-ray properties and to obtain a more
complete multiwavelength picture of the star-forming process.
metric gravitational potential in the bar channels gas toward the
Specifically, diffuse X-ray-emitting gas traces star formation in
center of the galaxy. Accretion stalls near the inner Lindblad
the disks (Tyler et al. 2004) and galactic winds in the halos
resonance forming a circumnuclear ring of gas and dust (Buta
(Wang et al. 2001; Strickland et al. 2004a, 2004b) of spiral gal-
Combes 1996). The density of this gas increases until the ring
axies, and we expect a similar correlation with the star-forming
becomes gravitationally unstable to collapse along its circum-
nuclear ring and inner ring in NGC 3351.
ference and several giant gas clouds form (Elmegreen 1994).
A description of the Chandra observations, data reduction
These clouds become hot spots of intense star formation. Pre-
methods, and properties of the discrete X-ray sources within
sumably, once enough gas has been converted to stars, the cloud
NGC 3351 are presented in x 2. In x 3 we investigate the mor-
remnants are dispersed by stellar winds and supernovae. The
entire process may repeat itself if accretion continues. Ultimately phology of the X-ray emission (both diffuse and pointlike) in the
circumnuclear region and compare it to star formation tracers
this depends on the stability of the bar itself, which can dissolve
observed at other wavelengths. We find that the likely source
if the core mass exceeds $5% of the total galaxy mass ( Norman
of X-ray emission from the circumnuclear ring is recent core-
et al. 1996). The dynamics of these structures is critical to ga-
collapse supernovae (SNe) rather than stellar winds from young
lactic bulge growth and hence evolution along the Hubble se-
stars and that the SN rate needed to account for the X-ray proper-
quence (e.g., Wyse et al. 1997; Elmegreen 2005), to forming and
ties is consistent with the recent star formation rate as deduced
sustaining central supermassive black holes (Wyse 2004; Ann
from the H luminosity. We show that X-ray emission extends in-
Thakur 2005), and to the physics behind the MBH -Ã relation
terior and exterior to the circumnuclear ring and that this emis-
( Ferrarese Merritt 2000).
sion is likely from an outflow—seen in projection—originating
NGC 3351 is an excellent example of a nearby (10.05 Mpc,
1 00 ¼ 49 pc; Graham et al. 1997), early-type barred spiral galaxy from the core of the galaxy. We discuss whether or not the out-
with a star-forming circumnuclear ring (Fig. 1). The bar is $4.6 kpc flow is confined and what implications this has on the dynamics
long and is surrounded by a 1 kpc wide inner ring. The circum- and evolution of the nuclear region in x 4.
nuclear ring is $0.5–0.7 kpc in diameter and is characterized
by a series of hot spots visible in UV (Colina et al. 1997), H 2. X-RAY OBSERVATIONS AND DATA
(Planesas et al. 1997; Bresolin Kennicutt 2002), near-infrared ANALYSIS OF NGC 3351
(Elmegreen et al. 1997), and optical images. Chandra Advanced CCD Imaging Spectrometer (ACIS) obser-
vations of NGC 3351 were obtained on 2005 February 8 (ObsID
1
Universities Space Research Association, NASA Marshall Space Flight 5929), 2005 May 8 (5930), and 2005 July 1 (5931). The duration of
Center, XD12, Huntsville, AL 35805.
2
each observation was approximately 39.5 ks, and the intervals be-
Department of Physics, University of Alabama, Huntsville, AL 35899. tween observations were 89 and 54 days, respectively. The location
3
Space Science Department, NASA Marshall Space Flight Center, XD12,
Huntsville, AL 35805. of the nucleus of NGC 3351 was offset 10 from the back-illuminated
4
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cam- CCD S3 aim point in all observations to center the galaxy on the
bridge, MA 02138. S3 detector. All observations were made in the full-frame timed
1030
2. Fig. 1.—Top: Three-color optical image of NGC 3351 adapted from the Sloan Digital Sky Survey ( blue, green, and red correspond to theg, r, and i bands, respectively).
North is up, and east is to the left. The image covers a 5 0 ; 5 0 area. The nuclear region is overexposed but contains a circumnuclear ring (see Fig. 2). The nuclear region is
surrounded by dark dust lanes that extend along the leading edge of the bar. The bar, appearing yellow in this image, is oriented at an angle $110 east of north. The bar
terminates at an inner ring (blue with dark dust lanes) of $6500 radius (3.2 kpc). Spiral arms extend beyond the inner ring. Bottom: Color-selected regions from the top panel
image chosen to show just the nuclear region, dust lanes, and inner ring.
3. 1032 SWARTZ ET AL.
exposure mode using the standard 3.2 s frame time, the stan- during each observation estimated by constructing light curves,
dard ACIS-S imaging mode chip set, and the faint telemetry grouped into 2 ks bins, and performing 2 tests against the con-
format. stant count rate hypothesis. None of the sources varied signifi-
Beginning with the Level 1 event lists, a CTI correction was ap- cantly according to this test (2 P 40 for 20 degrees of freedom
plied and pixel randomization was removed (using the Chandra [dof ] or P3 ). Results of these tests are indicated in column (8)
X-Ray Center’s CIAO ver. 3.2.1 tool acis_process_events). of Table 1 for the observation with the most significant vari-
The standard ASCA event grades were selected; a GTI filter was ability. (b) Variability over the intervals between observations
applied, and hot pixels, columns, and cosmic-ray afterglows were estimated by the statistical significance, V , of the difference in
removed to produce the final event lists for analysis. Three brief, the number of counts detected in the individual observations from
400 s, flares were detected near the end of ObsID 5929 with the mean of all three observations. Several sources varied signif-
count rates up to twice the mean background count rate over the icantly between observations according to this test (jV j k 3). The
entire observation. These were determined to have no impact on largest value of jV j among the three observations for each source
subsequent data analysis and were retained. is listed in column (9) of Table 1.
A fourth data set was constructed by co-adding the three in-
dividual observations using FTOOLS utilities.5 This requires a 2.2. Tests for the Pointlike Nature of the X-Ray Sources
coordinate translation of two of the data sets to register them to
The majority of the discrete X-ray sources detected in the NGC
the third data set and merging of the GTI extensions to produce
3351 field are expected to be bright individual X-ray binaries
the correct GTIs for the co-added data. The translations were com- (XRBs). Some SN remnants (SNRs) in NGC 3351, background
puted from the relative positions of a bright source near the aim
active galactic nuclei, and foreground stars are also expected to
point that was detected in all three observations. As a check of the
be in the field. The nuclear region of NGC 3351 also contains
absolute source locations, we compared X-ray source positions substantial amounts of hot diffuse gas, and we analyze this region
to Two Micron All Sky Survey (2MASS) All-Sky Data Release
in detail in x 3. In order to do so, we made a careful evaluation
catalog entries and found four sources with 2MASS counterparts.
of the sources within the circumnuclear region to distinguish
Positions of these sources are within 1 ACIS pixel (0B492 pixelÀ1)
between individual X-ray point sources and highly concentrated
of their 2MASS positions, and no further adjustments of the
knots of hot gas that may appear pointlike in the X-ray images.
Chandra astrometry was made.
Knots of hot gas should be relatively soft and steady X-ray sources
The central region of NGC 3351, the focus of the present
and may appear extended at the resolution of Chandra.
study, is located near the center of the back-illuminated S3 CCD The standard method used to test for source extent is to extract
in all three observations. Changes in instrument response and
azimuthally averaged radial profiles of sources and compare them
spacecraft orientation among the three observations were deter-
to the profiles of model point-spread functions (PSFs). The in-
mined to have minor effects on the X-ray data at this location. tensity and spatial inhomogeneity of the extended X-ray emission
Therefore, the fourth (co-added) data set is used extensively in
and the high density of sources in the nuclear region makes this
this work. Spectral response matrices and ancillary response files
technique impractical. Instead, we simultaneously fit circular
(generated using CIAO ver. 3.2.1 software, which accounts for
Gaussian models to the observed distribution of events associ-
contamination buildup) for the intermediate observation (ObsID
ated with the 12 sources in the inner 32 00 ; 32 00 region of NGC
5930) were used in the analysis of the co-added data.
3351 (indicated by asterisks in Table 1) with additional (circular
and elliptical) Gaussian models representing the underlying
2.1. Discrete X-Ray Sources background structure in this region. The positions, Gaussian
Analysis of the discrete X-ray source population observed in values, and normalizations were allowed to vary in the fitting.
the individual observations is presented elsewhere (D. A. Swartz Sources in the nuclear region with k 0B5 are considered ex-
et al. 2006, in preparation). Some of the X-ray properties of these tended sources.
sources are tabulated here for reference. Fifty-three discrete sources The results of this fitting and the observed X-ray colors and
were detected within the optical extent of NGC 3351 above a X-ray variability of the sources suggest that five of the sources in
signal-to-noise ratio of 2.4 and with at least a 4.5 significance the nuclear region are likely XRBs. Three nonvariable sources
above local background. The optical extent of NGC 3351 was are pointlike but are likely unresolved knots of hot gas based on
defined as the blue-light isophote at surface brightness level their soft X-ray spectra. One source has a (soft) pointlike com-
25.0 mag sÀ2 (major diameter D25 ¼ 7A41). The 4.5 signifi- ponent above more extended emission, which is best modeled as
cance was chosen after visual inspection of potential sources multiple overlapping sources (with slightly offset centroids and
within the high-background nuclear region to ensure that all ob- different widths). The three remaining sources in the nuclear re-
vious pointlike sources were cataloged (normally, 5 is used). gion are clearly soft, steady, and extended regions of thermal
Table 1 lists the X-ray properties of these sources. Columns (1)– emission. The seven sources judged in this way to be hot gas knots
(3) contain a unique source name and source coordinates, listed rather than XRB-like point sources are designated by the prefix k
in order of increasing right ascension ( R.A.). Columns (4)–(6) in column (1) of Table 1.
contain the background-subtracted counts (with errors) detected
in the co-added data set in three broad X-ray bands defined as 3. CIRCUMNUCLEAR STAR FORMATION
S (0.3–1.0 keV ), M (1.0–2.0 keV ), and H (2.0–8.0 keV ). Col- The X-ray image of NGC 3351 was examined to search for the
umn (7) lists the 0.3–8.0 keV luminosities estimated assuming a effects of the circumnuclear ring, bar, and inner ring on its X-ray
À ¼ 1:7 power-law photon index and the Galactic absorption col- properties. We found no significant enhancement of diffuse or
umn along the line of sight, NH ¼ 2:8 ; 1020 cmÀ2. Columns (8)– discrete X-ray emission in the bar or inner ring relative to the
(9) list two measures of X-ray source variability: (a) Variability surrounding regions. We therefore focus our analysis on the cen-
tral region of NGC 3351 for the remainder of this work. The
5
Available at http://heasarc.gsfc.nasa.gov/docs/software/ftools/ftools_ menu active circumnuclear region of NGC 3351 appears as the over-
.html. exposed core in the image of Figure 1.
5. 1034 SWARTZ ET AL. Vol. 647
Fig. 2.—Images of the central 32 00 ; 32 00 region of NGC 3351. Top, left to right: Spitzer MIPS 24, IRAC 5.8, and IRAC 4.6 m; 2MASS J; and CTIO 4.0 m Mosaic2 I.
Bottom, left to right: CTIO 4.0 m Mosaic2 continuum-subtracted H and B; Hubble F218W; and Chandra 0.5–8.0 keV. The last panel identifies the sources using the
unique names given in Table 1. Circles are 100 in radius and denote X-ray source positions. Ellipses denote the circumnuclear ring (see text for details).
3.1. General Morphology 3.2. X-Ray Morphology and Spectrum
The circumnuclear star-forming region of NGC 3351 appears The X-ray emission follows roughly the morphology seen at I
as an elliptical ring of hot spots at wavelengths shortward of the through UV. It is composed of several pointlike features on the
I band (Fig. 2). The emission at infrared wavelengths, in contrast, ring with additional diffuse emission both inside and outside of
is dominated by light from late-type stars and is concentrated at the ring. The emission outside the ring extends to about 800 pc
the galactic nucleus, although there is some structure associated from the nucleus to the northwest and to the southeast.
with the ring visible in higher resolution images (cf. Elmegreen Figure 3 displays the central 32 00 ; 32 00 region of NGC 3351.
et al. 1997). At the longer wavelengths sampled by Spitzer and in The left panel is a true-color image of the X-ray intensity show-
the CO bands (see Devereux et al. 1992; Planesas et al. 1997) the ing the extension of the diffuse emission and indicating that the
emission is localized to two lobes near the extrema of the ring pointlike concentrations or knots on the ring have similar spec-
major axis. tral properties as the more extended diffuse emission: namely, a
We parameterized the circumnuclear ring as an elliptical an- soft spectrum indicative of hot gas. The center panel is a compo-
nulus with a Gaussian cross section fitted to the highest resolution site of X-ray (blue), UV (green), and H (red) images showing
data available: the Hubble F218W UV image. When constrained that the emission features at different wavelengths are not nec-
to be centered on the nucleus, the best-fit position angle of this essarily coincident. Regions emitting in all three wave bands ap-
model is nearly perpendicular to the bar axis. This is expected if pear as bright white regions. The right-hand panel displays the
the ring traces the x2 orbits perpendicular to the bar (Contopoulos X-ray spectral hardness ratio (H À S )/T where T ¼ S þ M þ H
Papayannopoulos 1980). Therefore, we constrained the position (x 2.1) showing that most of the emission is soft (red in the fig-
angle to be P:A: ¼ 15 east of north (cf. Buta 1988). With these ure) including the pointlike sources judged to be knots of hot gas
two constraints the ring semimajor axis is a ¼ 300 pc and the in contrast to the hard XRB sources, which appear blue. Several
major-to-minor axis ratio is a/b ¼ 1:35. Because of azimuthal small regions also appear blue, indicating relatively hard spectral
inhomogeneities along the ring circumference, we do not consider colors. These may be weak XRBs below the detection limit or
the derived width of the Gaussian ring cross section to have par- simply Poisson noise. The more extended hard color region near
ticular physical significance, but it is convenient to choose a ring the north end of the ring major axis is likely a region of high ab-
half-width of r $ 100 pc to delineate the various regions of the star sorption as this region also has high JÀK colors and has low
formation zone in the discussion that follows. The ring model surface brightness (indicating absorption) in the optical images
along with 100 radius circles denoting the positions of all the de- (Fig. 2).
tected X-ray sources are overlaid on the images of Figure 2. For analysis of the hot gas component of the star-forming
In addition to the circumnuclear ring, dust lanes are visible regions of NGC 3351, we consider three spatial components: the
along the leading edge of the bar (rotation is counterclockwise; ring, the region interior to the ring, and the region exterior to the
Buta 1988; Rubin et al. 1975) that terminate near the extremities ring above a surface brightness of 0.5 X-ray events per pixel in the
of the major axis of the circumnuclear ring (see Fig. 1). The dust 0.3–3.0 keV band. The boundaries of these regions are shown in
lane along the eastern arm of the bar is stronger and is closer to the hardness ratio plot of Figure 3. The spectrum of each region
the observer than the dust lane on the western arm. The eastern was extracted from the co-added data after excluding the sources
dust lane is connected to a shorter dust lane that extends clock- labeled ‘‘s’’ in Table 1 (see Fig. 2), as these are all likely XRBs.
wise from the southern end of the ring toward the northeast. A background spectrum was chosen in a region away from the cir-
There is an abrupt edge to the circumnuclear emission that ap- cumnuclear source region but not too close to the edge of the CCD
pears to be coincident with this dust feature. This is most clearly array where the additional absorption due to molecular buildup
seen in the X-ray image of Figure 2 but is present in high-contrast on the ACIS filter is highest. We confine our analysis to the 0.3–
B, V, and H images as well. 3.0 keV energy range because there is no measurable flux above
6. No. 2, 2006 CIRCUMNUCLEAR STAR FORMATION IN NGC 3351 1035
Fig. 3.—Left: True-color X-ray image of the central 32 00 ; 32 00 region of NGC 3351. Red, green, and blue colors sample the S, M, and H bands (x 2.1). Center:
Combined X-ray (blue), H (red ), and UV (green) image of the same region. The X-ray image has been smoothed using the CIAO tool csmooth. Right: X-ray hardness
ratio (H À S )/T of the same region. Blue denotes relatively high spectral hardness (color bar depicts scale: ratio ! À1 as S ! T ). Pixels in the smoothed image with fewer
than 0.5 counts appear dark.
3.0 keV in the diffuse source spectra and the detector background
is high below 0.3 keV.
The spectra were fitted with a variety of models representative
of thermal plasmas or thermal plasma plus power-law compo-
nents (representing hot gas and unresolved XRBs, respectively)
using the XSPEC analysis tool. All models required a low hy-
drogen column density; we thus constrained this parameter to be
equal to or exceed the Galactic value, NH ¼ 2:8 ; 1020 cmÀ2, in
all trials. Results of the best-fitting single-temperature absorbed
VMEKAL models for the three spatial regions are listed in Table 2
(errors are 90% confidence ranges for a single interesting pa-
rameter). This is the best-fitting model to the observed spectra.
In particular, neither a power-law component nor multiple ther-
mal components are required by the data. The observed spectrum
and best-fitting model for the three regions are shown in Figure 4.
The results of the spectral fits show the three regions have
similar X-ray temperatures, as can be deduced qualitatively from
TABLE 2
Diffuse Plasma Properties
Parameters Interior Ring Exterior
Fit Parameters
Source counts........................ 657 1725 1328
þ4:5
NH (1020cmÀ2) ...................... 2.8 4:7À3:4 2.8
Te ( MK) ................................ 6:8þ0:6
À1:5
þ0:5
6:7À0:9 6:7þ0:3
À1:0
Normalizationa ...................... 3:2þ1:0
À0:9
þ5:6
8:5À2:8 8:5þ0:7
À1:6
Ob .......................................... 0:14þ0:28
À0:14 0:39þ0:26
À0:21 0:28þ0:17
À0:10
Mg ......................................... 0:17 Æ 0:17 0:60þ0:41
À0:26 0:21þ0:15
À0:11
Si ........................................... 0:27þ0:64
À0:23 0:39þ0:31
À0:22 0:03þ0:16
À0:03
Fe........................................... 0:18þ0:08
À0:06 0:15þ0:06
À0:05 0:10þ0:03
À0:01
LX/1038 ergs sÀ1 .................... 2.5 6.5 5.0
Derived Parametersc
V [ f ] (1063 cm3)................... 1.1 1.5 38.3
ne [ f À1/2] (cmÀ3) .................. 0.19 0.26 0.05
P/k [ f À1/2] (105 K cmÀ3) ..... 26.0 35.0 6.7
Mx [ f + 1/2] (105 M) ............ 2.5 4.5 22.9
Eth [ f +1/2] (1053 ergs)........... 6.0 10.8 54.3
tc [ f +1/2] ( Myr) .................... 77 53 348 Fig. 4.—X-ray spectrum and best-fitting absorbed VMEKAL model (solid
R line) from the region interior to the circumnuclear ring (top), within the ring
a
K ¼ (10À9 /4D 2 ) ne np dV (see text). (middle), and exterior to the ring (bottom). Fit parameters are listed in Table 2. For
b
Abundance relative to solar values from Anders Grevesse (1989). display purposes, up to 10 adjacent energy bins have been combined to achieve a
c
Volume filling factor scaling in brackets (see text). significant detection at least as large as 5 .
7. 1036 SWARTZ ET AL. Vol. 647
inspection of the left panel of Figure 3. The bulk of the X-ray thermal energy in the ring is $1054 ergs. If 1051 ergs of kinetic
flux, however, comes from the circumnuclear ring. The highest energy is released by each SN, then of order 1000 SN events are
abundances relative to solar are for the low-Z metals indicative needed to produce the thermal energy assuming a high conver-
of enhancement by core-collapse SNe. There may be a slight en- sion efficiency. At an age of $4–6 Myr, only stars more massive
hancement of these intermediate-mass elements in the ring rel- than about 40 M have become SNe. For a Salpeter IMF with an
ative to the other two regions considered, but the uncertainties upper-mass cutoff of 100 M , roughly 1400 stars more massive
are large. than 40 M are produced in a starburst of total mass 106 M
The spectral model normalization parameter, K, is propor-
R (Leitherer et al. 1999). The thermal emission can therefore be
tional to the emission integral, ne nH dV $ n2 f V , where the
e accounted for by SNe from massive stars in a starburst of total
integral is over a volume, V, f represents a volume filling factor mass $106 M . If the stars in the X-ray-emitting regions are
such that the actual volume occupied by X-ray-emitting gas is older than 4–6 Myr, then stars less massive than 40 M are also
f V , and the assumption has been made that the ion and electron contributing to the SNe population and a starburst of less mass is
number densities ni and ne , respectively, are equal, which is required (or, equivalently, a lower efficiency converting kinetic
roughly true for a hydrogen-dominated hot plasma with ni $ nH . to thermal energy is needed).
Hence, the electron density in the X-ray-emitting gas can be The actual conditions in the circumnuclear ring are probably
derived from the spectral model normalizations and knowledge intermediate between the instantaneous starburst and the con-
of the emission geometry. tinuous star formation scenarios: the fact that the H and X-ray
We assume that the ring is a circular torus of volume hot spots are not colocated and that they measure different but
Vr ¼ 2 2 ar 2 and that the region interior to the ring is a sphere of overlapping stages in the starburst evolution suggests that they
volume Vi ¼ (4/3)(a À r) 3 using the ring parameters of x 3.1. are not coeval. If star formation in the ring is triggered by frag-
The geometry is more uncertain for the region exterior to the mentation into collapsing clouds driven by gravitational insta-
ring. If the emission is extended above the bulge of the galaxy, as bilities ( Elmegreen 1994), then it is natural to expect some
may be the case (x 3.4), then the volume may be quite large. For cloud-to-cloud variation in their physical properties and hence
simplicity, however, we represent it as an oblate spheroid defined some variation in the onset and evolution of their star formation
by the ellipse that most closely matches the surface brightness processes.
contour at 0.5 events per pixel: R ¼ 780 pc is the semimajor axis,
and the major-to-minor axis ratio is 1.43. The volume of the emis- 3.3.1. X-Ray Hot Spots
sion region exterior to the ring is then Vo ¼ (4/3)R2 (0:7R) À We can compare the properties of the individual H hot spots
Vr À Vi . to those of the X-ray knots. Planesas et al. (1997) estimated the
From ne and the flux-weighted mean temperatures Te we can mass of gas in several hot spots from the observed H lumi-
estimate the hot gas pressure P/k ¼ 2ne Te , mass Mx ¼ ne mp V , nosity and assuming an electron density of 250 cmÀ3. These
where mp is the proton mass and ¼ 1:4 is the mass per proton, masses range from 5 ; 103 to 4 ; 104 M for a total mass of 3:5 ;
thermal energy Eth ¼ 3ne VkTe, and the cooling time tc ¼ Eth /LX 105 M in the H -emitting ring gas. The conditions in the in-
of the X-ray-emitting plasma as listed in Table 2. These derived dividual X-ray-emitting knots can be estimated by scaling from
quantities are all functions of the volume filling factor as indi- the mean density in the ring as derived from the X-ray spectrum:
cated in Table 2. 2 2
the density in the kth knot is then nk ¼ ne ( f /fk )(V /Vk )(Ck /C ),
where C is the number of X-ray counts and it is assumed the
3.3. The Circumnuclear Ring
knots and the surrounding gas have the same plasma tempera-
The most intense recent star formation in NGC 3351 is traced ture. Assuming the knots are spherical with radii equal to the
by the H emission in the circumnuclear ring. The X-ray data Gaussian half-widths deduced from our source fitting as de-
are used in this section to help constrain the age of the star forma- scribed in x 2.2 and that the number of counts in the knots is
tion activity and the mass involved in the star formation process. given by the Gaussian normalizations,6 then the derived prop-
Bresolin Kennicutt (2002) estimated the age of the H hot erties of the knots are as listed in Table 3. (Source k34 is not on
spots at 6 Myr from H
8. and He i k5876 equivalent widths as- the ring and is not included in Table 3.)
suming instantaneous bursts of star formation. Colina et al. The masses of the X-ray knots are similar to the H hot spot
(1997) found ages of 4–5 Myr for the entire circumnuclear re- masses, but the pressures are several times higher (H hot spot
gion from IUE spectra. On the other hand, Planesas et al. (1997) pressures are P/k $ 5 ; 106 K cmÀ3 for temperatures of 104 K
estimated the total ionizing flux responsible for the hot spots to and densities 250 cmÀ3). This again suggests that the principal dif-
be $1053 photons sÀ1. Such a high flux level is only produced ference between H hot spots and X-ray knots is the age of their
within the first $2 Myr after a starburst (Sternberg et al. 2003) starbursts. The older X-ray-emitting regions are being powered
because the massive early O stars supplying the flux quickly die off by SNe, and the resulting overpressures will force hot tenuous
in SN explosions. The ionizing flux could be sustained for longer gas away from surrounding cold, dense regions of the ring. The
periods if star formation is continuous at a rate of $0.4 M yrÀ1 thermal energies of the individual knots are of order 1052 ergs,
(Sternberg et al. 2003). Planesas et al. (1997) estimated the total implying of order 10 SNe power each hot spot.
star formation rate in the H hot spots is 0.25 M yrÀ1. Another source of X-ray emission in the ring (and elsewhere)
The X-ray emission in the star-forming region is predomi- is accretion onto compact objects. There is one bright and var-
nantly produced by SNe interacting with the ionized shells formed iable X-ray source, s24, detected in the circumnuclear ring. The
by massive star winds (Weaver et al. 1977; Chu Mac Low source luminosity is $1038 ergs sÀ1 at its peak. Such high lu-
1990; Shull Saken 1995). Thus, the X-ray emission traces a minosities imply high accretion rates, which occur in Roche lobe
slightly later phase of the evolution of a starburst, $10 Myr, overflow systems when the companion ascends the giant branch.
when the many late O and B stars are dying off. Individual SNe
can be copious X-ray emitters (e.g., Immler Lewin 2003), but 6
For sources with multiple Gaussian components, the largest model
the emission is a sensitive function of the ambient conditions in value is given, and the number of counts listed is the sum over all components.
which the SNR forms. From the X-ray emission, we estimate the See x 2.2.
9. No. 2, 2006 CIRCUMNUCLEAR STAR FORMATION IN NGC 3351 1037
TABLE 3
X-Ray Properties of Circumnuclear Hot Spots
Radius Volume ne P/k MX LX Eth
Source Number (arcsec) (1060 cmÀ3) Counts (cmÀ3) (107 K cmÀ3) (103 M) (1037 ergs sÀ1) (1051 ergs)
k25........................ 0.4 0.9 65.9 2.1 2.8 2.2 2.5 5.2
k26........................ 0.6 3.1 62.4 1.1 1.5 4.0 2.4 9.5
k27........................ 1.0 14.2 278.0 1.1 1.5 18.3 10.6 43.3
k30........................ 0.8 7.3 94.1 0.9 1.2 7.7 3.6 18.2
k31........................ 0.3 0.4 27.3 2.0 2.7 0.9 1.0 2.2
k33........................ 1.3 31.2 187.6 0.6 0.8 21.9 7.2 51.9
Total: ... ... ... ... ... 55.0 27.3 ...
Statistically, the companion is probably a late O or B star (as is from material confined by this dust. The fact that this dust lane
there are more of these stars available and they spend longer extends counterclockwise from the dust lane along the leading
periods off the zero-age main sequence than do more massive edge of the bar suggests it may have formed out of the outflow-
stars). Thus, the age of the variable XRB source on the ring is of ing gas as it cooled instead of being entrained along the leading
order 10–50 Myr for a B0–B3 companion, and it represents a later edge of bar orbits. The lack of significant absorption affecting the
phase in the evolution of the ring compared to the X-ray hot spots. X-ray spectrum (x 3.2) means that there is little or no cold material
Grimm et al. (2003) have empirically estimated the X-ray lu- above the X-ray-emitting outflow.
minosity and number of bright high-mass XRBs as a function of We extracted a radial profile of the diffuse emission, centered
(continuous) star formation rate. For the rate of 0.25 M yrÀ1 in on the nucleus, after omitting discrete sources and fitted expo-
the circumnuclear ring (Planesas et al. 1997), Grimm et al. pre- nential and power-law models to the data. The best-fitting model
dicted 1–2 XRBs with luminosity 1038 ergs sÀ1 and 5–8 with is an exponential, / exp (Àr/H ), where H $ 170 pc is the scale
luminosity 1037 ergs sÀ1. We only detect the one (variable) length, resulting in a goodness of fit 2 ¼ 113:1 for 87 dof. For
XRB in the circumnuclear ring at $1038 ergs sÀ1, but there could comparison, the best-fitting power law has 2 ¼ 120:9 for 87 dof
be several weak sources near (but not much above) 1037 ergs sÀ1 for an index of À2.9.
that have escaped detection. Thus, the number of XRBs detected Strickland et al. (2004a) argued that an exponential profile
in the ring is consistent with expectations for continuous star (perpendicular to the plane) indicates a galactic fountain model
formation at a moderate rate. for halo gas, i.e., the X-ray surface brightness profile follows the
underlying density distribution and the hot gas is filamentary in
3.4. X-Ray Emission External to the Ring structure. In contrast, models with emission from a volume filling
In contrast to the emission from the circumnuclear ring, there component such as occurs if substantial mass is injected into the
are no indications of current or recent star formation accompa- halo by the outflow (e.g., Suchkov et al. 1996) produce power-law
nying the X-ray emission from regions exterior to the ring. Here X-ray surface brightness profiles with index $À3 (Chevalier
we show that the most likely source of this emission is an outflow Clegg 1985).
from the ring itself. 3.5. X-Ray Emission Internal to the Ring (the Nucleus)
Outflow or breakout from pressure-confined regions into the
halos of spiral and starburst galaxies is a common phenomenon As with the region exterior to the ring, we surmise the bulk of
(e.g., Strickland et al. 2002, 2004a; Tyler et al. 2004; Wang the X-ray emission observed interior to the ring originated in the
2005). When viewed edge-on, galactic outflows are rather ten- ring and traces an outflow.
uous with the surface brightness decreasing exponentially with
increasing height above the galaxy plane. The outflow may be
confined by ambient halo material (which may be accumulated
gas from earlier outflows) as in, e.g., NGC 3077 (Ott et al. 2003)
or may be escaping into the intragalactic medium as in strong
starburst systems (Strickland et al. 2004a, 2004b).
The 40 inclination of NGC 3351 makes it difficult to assess
whether the observed emission is confined to the plane of the gal-
axy or extends significantly above it. Inspection of Figure 3 sug-
gests a sharp edge to the distribution of X-ray emission southeast
of the circumnuclear ring but a more gradual decrease in the sur-
face brightness to the northwest. If the emitting gas is rising out
of the circumnuclear ring in a conical pattern, the sharp feature
could be tracing the edge of the cone of material expelled nearly
along our line of sight, whereas the movement of material from Fig. 5.—Schematic representation of a possible outflow geometry for NGC
the far side of the ring is more nearly perpendicular to our line of 3351. The galaxy is viewed from the left at an inclination of 40 , and the plane of
sight. This is shown schematically in Figure 5. the galaxy is perpendicular to the page. A conical flow of hot gas out of the plane
On the other hand, there is a dark region to the southeast of the is depicted by arrows. Cold gas within the plane of the galaxy is shown with
circumnuclear ring visible in the Sloan image of Figure 1. This heavy shading (only one face of the galaxy is shown). Outflow material that has
cooled and condensed out of the flow could be falling back onto the disk. This is
feature is likely a dust lane in or near the plane of the galaxy. The depicted as shaded regions on the edge of the outflow cone. The lack of X-ray-
edge of this dust lane coincides with the sharp edge in the X-ray absorbing material along our line of sight suggests there is little or no cold
surface brightness distribution, implying that the X-ray emission material above the X-ray-emitting outflow.
10. 1038 SWARTZ ET AL. Vol. 647
There is little evidence of concentrated star formation in this (1997) estimated an H2 mass of 3:5 ; 108 M within 700 pc of
region. The surface brightness at near-infrared wavelengths does the center of NGC 3351 from radio measurements of CO 2 ! 1
peak at the center of the galaxy, but this emission traces the older emission. Devereux et al. (1992) obtained about half this value
underlying bulge population of stars. There is a modest amount for the molecular mass within the circumnuclear ring of NGC
of H emission at the center, which could indicate a buildup of 3351, a region of equivalent area as sampled by Planesas et al.
gas there. Planesas et al. (1997) estimated that the mass of H- (1997). The mean number density of molecular gas within an
emitting gas at the center of the galaxy is 5000 M . How this gas oblate spheroid of dimensions 900 ; 300 pc (following Devereux
is excited is unclear. There is no evidence for heating by young et al. 1992) is then 7 H2 cmÀ3. However, the CO emission mor-
stars in situ. Possibly, gas escaping the ring but still confined to phology is similar to the far-infrared emission shown in Figure 2;
the bulge gravitational potential is converging at the nucleus and namely, it is concentrated in two regions near the ends of the ring
being reheated by shocks. Another explanation is a nuclear su- major axis. Thus, we estimate the molecular gas density at the nu-
permassive black hole is exciting its surroundings. cleus to be $10% of the mean (consistent with our estimated
No individual X-ray sources are detected interior to the ring. extinction toward the nucleus)7 or 0.35–0.70 H2 cmÀ3.
The 90% confidence upper limit to the X-ray emission from the Thus, the amount of material available for accretion onto any
nucleus is LX ¼ 5:5 ; 1036 ergs sÀ1 in the 0.5–8.0 keV band, compact object at the core of NGC 3351 is substantial. The
assuming a À ¼ 1:7 power-law index and the Galactic column observed upper limit to the X-ray luminosity from the nucleus
density of 2:8 ; 1020 cmÀ2. This estimate allows for uncertainty implies the radiative efficiency of this accretion must be extremely
in the registration of the X-ray image and in the absolute position low (consistent with the prediction of advection-dominated accre-
of the nucleus by assuming the brightest pixel in a 200 radius tion flow models) and the accretion rate must be P10% of the
about the nominal center of NGC 3351 is the nucleus and using a classical Bondi rate.
100 source extraction radius.
The Sloan Digital Sky Survey obtained an optical spectrum of 4. DISCUSSION
the nucleus of NGC 3351. It shows Balmer emission lines but We have analyzed the X-ray emission from the central region
lacks most other features characteristic of active galactic nuclei. of NGC 3351. The general picture that emerges is that X-rays are
From the continuum shape of this spectrum and from the produced in regions of intense recent star formation within a cir-
2MASS JÀK colors, we estimate the extinction in the nucleus of cumnuclear ring and that some hot gas is escaping into surround-
NGC 3351 to be E(BÀV ) $ 0:3. This corresponds to a hydrogen ing regions.
column density of 2 ; 1021 cmÀ2 or about 10 times the Galactic The circumnuclear ring is also a strong H and UV emitter.
value. This would increase the estimated X-ray upper limit lu- The emission morphology can be understood if star formation is
minosity to 7 ; 1036 ergs sÀ1 and the unabsorbed (intrinsic) X- occurring through a series of instantaneous bursts at various lo-
ray luminosity to about 1037 ergs sÀ1. For comparison, Planesas cations around the ring with differing amounts of extinction.
et al. (1997) estimated the H luminosity at the center of the Then the UV, H, and X-ray emission trace different stages in
galaxy as 3:5 ; 1038 ergs sÀ1. Thus, the H emission from the the evolution of the underlying stellar populations beginning
nuclear region is far more than can be accounted for by X-ray with UV and H emission from the youngest and most massive
photoionization. stars and culminating in SN-powered X-ray emission about 10 Myr
Dong De Robertis (2006) estimated that the central mass after the starbursts. This explains the general lack of spatial co-
concentration in NGC 3351 is $107 M using the correlation incidence among individual UV, H , and X-ray hot spots if they
between MBH and K-band luminosity (after rescaling to a dis- were formed at different times and the UV hot spots are relatively
tance of 10 Mpc to NGC 3351). This is consistent with the unobscured. The few regions that are radiating strongly in all three
estimate of Sarzi et al. (2002) from measured emission-line wave bands are probably undergoing a more extended star forma-
widths using the MBH -Ã relation and the estimate from the tion episode lasting of order 10 Myr or longer.
correlation between MBH and the Sersic index of the optical Of order 1000 SNe are needed to produce the X-ray lumi-
surface brightness profile (Graham et al. 2003). nosity emanating from the ring. Since the cooling timescale of
The Bondi rate of accretion of hot gas (kT $ 0:5 keV ) for an this gas is long, 50 f 1/2 Myr (where f is the volume filling factor),
object of this mass is MB $ 2 ; 10À5 (ne /cmÀ3 ) M yrÀ1 or about
˙ the SN rate need only be 2 ; 10À5 f À1/2 yrÀ1. This is easily main-
˙
10À4 of the Eddington accretion rate MEdd . The hot gas density in tained by continuous star formation at a rate of 0.25–0.40 M yrÀ1
the nuclear region was estimated in x 3.2 as ne $ 0:2f À1/2 cmÀ3. or (after about 10 Myr) by a starburst of $106 M for reasonable
Assuming a 10% radiative efficiency, the luminosity expected from values of f k10À4 . The thermal energy contained in the hot gas,
Bondi accretion is therefore L ¼ 0:1MB c 2 $ 3 ; 10 40 ergs sÀ1 or
˙ Eth $ f 1/2 1054 ergs, is consistent with being generated by 1000 SNe
$2 ; 10À5 of the Eddington luminosity, LEdd . If, instead, accre- each producing 1051 ergs in total energy if the conversion from
tion is advection dominated, then the radiative efficiency is much kinetic to thermal energy is reasonably efficient.
lower and the accretion rate may be an order of magnitude The hot gas observed outside of the circumnuclear ring was
lower than the classical Bondi rate (Narayan 2002). From the self- likely produced within the ring and is escaping from pressure-
similar solution of Narayan Yi (1994), we expect a nuclear confined clouds toward lower pressure regions perhaps in and
˙ ˙
luminosity $(MB /MEdd ) 2 LEdd $ 10À8 LEdd $ 10 37 ergs sÀ1. A likely above the plane of the galaxy.
similar order-of-magnitude estimate for the intrinsic nuclear X-ray Some of the gas escaping the circumnuclear ring undoubtedly
emission can be obtained from the more detailed advection model makes its way into the sphere of influence of the nuclear su-
of Merloni et al. (2003). permassive black hole. We estimate there is ample fuel available
There is also warm ( H -emitting) and cold gas present in the at the center of the galaxy to power active galactic nucleus ac-
core of NGC 3351. Planesas et al. (1997) estimated that the H tivity simply through Bondi accretion. However, the upper limit
luminosity at the center of the galaxy as 3:5 ; 1038 ergs sÀ1. We of $1037 ergs sÀ1 to the X-ray luminosity of any point source at
estimate that the size of the H emission region is P30–50 pc
from the H image, i.e., at or below the limit of resolution. Thus, 7
The CO measurements correspond to nH $ 1:4 ; 1022 cmÀ2, assuming a
the density of warm gas is at least ne ¼ 8 cmÀ3. Planesas et al. uniform distribution.
11. No. 2, 2006 CIRCUMNUCLEAR STAR FORMATION IN NGC 3351 1039
the nucleus suggests that the accretion is highly inefficient if the The total estimated mass of hot gas in the ring and surrounding
central object has a mass of 107 M as has been reported. regions is $3 ; 106 M . This is comparable to the estimated
Evidence is ambiguous whether the outflowing hot gas is es- mass of warm, H -emitting gas but only about 1% of the mass
caping as a galactic fountain or is confined by surrounding cold in molecular gas and 0.1% of the dynamical mass within the cen-
gas as a superbubble. No overlying layer of cold gas is evident in tral $1 kpc of NGC 3351. Thus, even if the hot gas escapes al-
the X-ray spectrum; however, there is a dust lane to the southeast together it represents only a small fraction of the total mass
of the X-ray-emitting region that could confine the gas there as it budget: there is a substantial reservoir of potentially star-forming
appears to coincide with a decrease in the X-ray surface bright- (cold) material available, enough to maintain star formation at a
ness. Perhaps the gas is rising away from the plane of the galaxy moderate rate for 100 Myr or longer within the inner regions of
in a conical pattern: freely expanding vertically but constrained NGC 3351.
from moving laterally by ambient cold gas. If the flow at the
edges of the cone is roughly tangential to the cold gas, then
strong (shearing) shocks need not form and no limb-brightening
need occur. Support for this research was provided in part by NASA/
The estimated cooling timescale for this gas is as long as Chandra Award Number GO5-6089A issued by the Chandra
350 Myr (scaling as f 1/2 and also dependent on the uncertain emis- X-Ray Observatory Center, which is operated by the Smithso-
sion volume). Unless it is pressure confined, this gas could travel nian Astrophysical Observatory for and on behalf of NASA
large distances before cooling and presumably falling back onto under contract NAS 8-03060. This work made use of observa-
the galaxy in the form of molecular clouds. Although seen at a tions made with the Spitzer Space Telescope, which is operated
40 inclination, there is no evidence that the gas has traveled more by the Jet Propulsion Laboratory, California Institute of Technol-
than about 500 pc from the ring. That is, fallback does not extend ogy, under a contract with NASA and of related ground-based
as far as the ends of the bar nor to the inner ring some 3 kpc away. data obtained as part of the Spitzer Legacy Science project SINGS
What the source of material that produces the dust lanes visible (Kennicutt et al. 2003), to which we are all greatly indebted. D. A. S.
along the leading edge of the bar is and how it is maintained or thanks A. Bykov for fruitful discussions and references on X-ray
replenished is not revealed by the X-ray data. emission from star-forming clouds.
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