Dynamical evidence accumulated over the
past 20 years has convinced astronomers that luminous matter
in a spiral galaxy constitutes no more than 10% of the mass of
a galaxy. An additional 90% is inferred by its gravitational
effect on luminous material. Here I review recent observations
concerning the distribution of luminous and nonluminous
matter in the Milky Way, in galaxies, and in galaxy clusters.
Observations of neutral hydrogen disks, some extending in
radius several times the optical disk, confirm that a massive
dark halo is a major component of virtually every spiral. A
recent surprise has been the discovery that stellar and gas
motions in ellipticals are enormously complex. To date, only for
a few spheroidal galaxies do the velocities extend far enough to
probe the outer mass distribution. But the diverse kinematics
of inner cores, peripheral to deducing the overall mass distribution,
offer additional evidence that ellipticals have acquired
gas-rich systems after initial formation. Dynamical results are
consistent with a low-density universe, in which the required
dark matter could be baryonic. On smallest scales of galaxies
[10 kiloparsec (kpc); H. = 50 kmsec'lmegaparsec'11 the
luminous matter constitutes only 1% of the closure density. On
scales greater than binary galaxies (i.e., .100 kpc) all systems
indicate a density -10% of the closure density, a density
consistent with the low baryon density in the universe. If
large-scale motions in the universe require a higher mass
density, these motions would constitute the first dynamical
evidence for nonbaryonic matter in a universe of higher
density.
A highly magnetized twin-jet base pinpoints a supermassive black holeSérgio Sacani
Supermassive black holes (SMBH) are essential for the production of jets in radio-loud active galactic nuclei (AGN). Theoretical
models based on (Blandford & Znajek 1977, MNRAS, 179, 433) extract the rotational energy from a Kerr black hole, which could
be the case for NGC1052, to launch these jets. This requires magnetic fields on the order of 103 G to 104 G. We imaged the vicinity
of the SMBH of the AGN NGC1052 with the Global Millimetre VLBI Array and found a bright and compact central feature that is
smaller than 1.9 light days (100 Schwarzschild radii) in radius. Interpreting this as a blend of the unresolved jet bases, we derive the
magnetic field at 1 Schwarzschild radius to lie between 200 G and 8:3 104 G consistent with Blandford & Znajek models.
PROBING FOR EVIDENCE OF PLUMES ON EUROPA WITH HST/STISSérgio Sacani
Roth et al. (2014a) reported evidence for plumes of water venting from a southern high latitude
region on Europa – spectroscopic detection of off-limb line emission from the dissociation
products of water. Here, we present Hubble Space Telescope (HST) direct images of Europa in
the far ultraviolet (FUV) as it transited the smooth face of Jupiter, in order to measure absorption
from gas or aerosols beyond the Europa limb. Out of ten observations we found three in which
plume activity could be implicated. Two show statistically significant features at latitudes similar
to Roth et al., and the third, at a more equatorial location. We consider potential systematic
effects that might influence the statistical analysis and create artifacts, and are unable to find any
that can definitively explain the features, although there are reasons to be cautious. If the
apparent absorption features are real, the magnitude of implied outgassing is similar to that of the
Roth et al. feature, however the apparent activity appears more frequently in our data.
The ASTRODEEP Frontier Fields catalogues II. Photometric redshifts and rest f...Sérgio Sacani
Aims. We present the first public release of photometric redshifts, galaxy rest frame properties and associated magnification values
in the cluster and parallel pointings of the first two Frontier Fields, Abell-2744 and MACS-J0416. The released catalogues aim to
provide a reference for future investigations of extragalactic populations in these legacy fields: from lensed high-redshift galaxies to
cluster members themselves.
Methods.We exploit a multiwavelength catalogue, ranging from Hubble Space Telescope (HST) to ground-based K and Spitzer IRAC,
which is specifically designed to enable detection and measurement of accurate fluxes in crowded cluster regions. The multiband
information is used to derive photometric redshifts and physical properties of sources detected either in the H-band image alone, or
from a stack of four WFC3 bands. To minimize systematics, median photometric redshifts are assembled from six dierent approaches
to photo-z estimates. Their reliability is assessed through a comparison with available spectroscopic samples. State-of-the-art lensing
models are used to derive magnification values on an object-by-object basis by taking into account sources positions and redshifts.
Results. We show that photometric redshifts reach a remarkable 3–5% accuracy. After accounting for magnification, the H-band
number counts are found to be in agreement at bright magnitudes with number counts from the CANDELS fields, while extending
the presently available samples to galaxies that, intrinsically, are as faint as H 32 33, thanks to strong gravitational lensing. The
Frontier Fields allow the galaxy stellar mass distribution to be probed, depending on magnification, at 0.5–1.5 dex lower masses with
respect to extragalactic wide fields, including sources at Mstar 107–108 M at z > 5. Similarly, they allow the detection of objects
with intrinsic star formation rates (SFRs) >1 dex lower than in the CANDELS fields reaching 0.1–1 M=yr at z 6–10.
A highly magnetized twin-jet base pinpoints a supermassive black holeSérgio Sacani
Supermassive black holes (SMBH) are essential for the production of jets in radio-loud active galactic nuclei (AGN). Theoretical
models based on (Blandford & Znajek 1977, MNRAS, 179, 433) extract the rotational energy from a Kerr black hole, which could
be the case for NGC1052, to launch these jets. This requires magnetic fields on the order of 103 G to 104 G. We imaged the vicinity
of the SMBH of the AGN NGC1052 with the Global Millimetre VLBI Array and found a bright and compact central feature that is
smaller than 1.9 light days (100 Schwarzschild radii) in radius. Interpreting this as a blend of the unresolved jet bases, we derive the
magnetic field at 1 Schwarzschild radius to lie between 200 G and 8:3 104 G consistent with Blandford & Znajek models.
PROBING FOR EVIDENCE OF PLUMES ON EUROPA WITH HST/STISSérgio Sacani
Roth et al. (2014a) reported evidence for plumes of water venting from a southern high latitude
region on Europa – spectroscopic detection of off-limb line emission from the dissociation
products of water. Here, we present Hubble Space Telescope (HST) direct images of Europa in
the far ultraviolet (FUV) as it transited the smooth face of Jupiter, in order to measure absorption
from gas or aerosols beyond the Europa limb. Out of ten observations we found three in which
plume activity could be implicated. Two show statistically significant features at latitudes similar
to Roth et al., and the third, at a more equatorial location. We consider potential systematic
effects that might influence the statistical analysis and create artifacts, and are unable to find any
that can definitively explain the features, although there are reasons to be cautious. If the
apparent absorption features are real, the magnitude of implied outgassing is similar to that of the
Roth et al. feature, however the apparent activity appears more frequently in our data.
The ASTRODEEP Frontier Fields catalogues II. Photometric redshifts and rest f...Sérgio Sacani
Aims. We present the first public release of photometric redshifts, galaxy rest frame properties and associated magnification values
in the cluster and parallel pointings of the first two Frontier Fields, Abell-2744 and MACS-J0416. The released catalogues aim to
provide a reference for future investigations of extragalactic populations in these legacy fields: from lensed high-redshift galaxies to
cluster members themselves.
Methods.We exploit a multiwavelength catalogue, ranging from Hubble Space Telescope (HST) to ground-based K and Spitzer IRAC,
which is specifically designed to enable detection and measurement of accurate fluxes in crowded cluster regions. The multiband
information is used to derive photometric redshifts and physical properties of sources detected either in the H-band image alone, or
from a stack of four WFC3 bands. To minimize systematics, median photometric redshifts are assembled from six dierent approaches
to photo-z estimates. Their reliability is assessed through a comparison with available spectroscopic samples. State-of-the-art lensing
models are used to derive magnification values on an object-by-object basis by taking into account sources positions and redshifts.
Results. We show that photometric redshifts reach a remarkable 3–5% accuracy. After accounting for magnification, the H-band
number counts are found to be in agreement at bright magnitudes with number counts from the CANDELS fields, while extending
the presently available samples to galaxies that, intrinsically, are as faint as H 32 33, thanks to strong gravitational lensing. The
Frontier Fields allow the galaxy stellar mass distribution to be probed, depending on magnification, at 0.5–1.5 dex lower masses with
respect to extragalactic wide fields, including sources at Mstar 107–108 M at z > 5. Similarly, they allow the detection of objects
with intrinsic star formation rates (SFRs) >1 dex lower than in the CANDELS fields reaching 0.1–1 M=yr at z 6–10.
The shadow _of_the_flying_saucer_a_very_low_temperature_for_large_dust_grainsSérgio Sacani
Os astrónomos usaram o ALMA e os telescópios do IRAM para fazer a primeira medição direta da temperatura dos grãos de poeira grandes situados nas regiões periféricas de um disco de formação planetária que se encontra em torno de uma estrela jovem. Ao observar de forma inovadora um objeto cujo nome informal é Disco Voador, os astrónomos descobriram que os grãos de poeira são muito mais frios do que o esperado: -266º Celsius. Este resultado surpreendente sugere que os modelos teóricos destes discos precisam de ser revistos.
Uma equipa internacional liderada por Stephane Guilloteau do Laboratoire d´Astrophysique de Bordeaux, França, mediu a temperatura de enormes grãos de poeira que se encontram em torno da jovem estrela 2MASS J16281370-2431391 na região de formação estelar Rho Ophiuchi, a cerca de 400 anos-luz de distância da Terra.
Esta estrela encontra-se rodeada por um disco de gás e poeira — chamado disco protoplanetário, uma vez que se encontra na fase inicial da formação de um sistema planetário. Este disco é visto de perfil quando observado a partir da Terra e a sua aparência em imagens no visível levou a que se lhe desse o nome informal de Disco Voador.
Os astrónomos utilizaram o ALMA para observar o brilho emitido pelas moléculas de monóxido de carbono no disco da 2MASS J16281370-2431391. As imagens revelaram-se extremamente nítidas e descobriu-se algo estranho — em alguns casos o sinal recebido era negativo. Normalmente um sinal negativo é fisicamente impossível, mas neste caso existe uma explicação, que leva a uma conclusão surpreendente.
Detection of solar_like_oscillations_in_relies_of_the_milk_way_asteroseismolo...Sérgio Sacani
Asteroseismic constraints on K giants make it possible to infer radii, masses and ages of tens
of thousands of field stars. Tests against independent estimates of these properties are however
scarce, especially in the metal-poor regime. Here, we report the detection of solar-like
oscillations in 8 stars belonging to the red-giant branch and red-horizontal branch of the globular
cluster M4. The detections were made in photometric observations from the K2 Mission
during its Campaign 2. Making use of independent constraints on the distance, we estimate
masses of the 8 stars by utilising different combinations of seismic and non-seismic inputs.
When introducing a correction to the Δν scaling relation as suggested by stellar models, for
RGB stars we find excellent agreement with the expected masses from isochrone fitting, and
with a distance modulus derived using independent methods. The offset with respect to independent
masses is lower, or comparable with, the uncertainties on the average RGB mass
(4 − 10%, depending on the combination of constraints used). Our results lend confidence to
asteroseismic masses in the metal poor regime. We note that a larger sample will be needed
to allow more stringent tests to be made of systematic uncertainties in all the observables
(both seismic and non-seismic), and to explore the properties of RHB stars, and of different
populations in the cluster.
Observed glacier and volatile distribution on Pluto from atmosphere–topograph...Sérgio Sacani
Pluto has a variety of surface frosts and landforms as well as a
complex atmosphere1. There is ongoing geological activity related
to the massive Sputnik Planum glacier, mostly made of nitrogen (N2)
ice mixed with solid carbon monoxide and methane2, covering the
4-kilometre-deep, 1,000-kilometre-wide basin of Sputnik Planum1,3
near the anti-Charon point. The glacier has been suggested to arise
from a source region connected to the deep interior, or from a sink
collecting the volatiles released planetwide1. Thin deposits of N2
frost, however, were also detected at mid-northern latitudes and
methane ice was observed to cover most of Pluto except for the
darker, frost-free equatorial regions2. Here we report numerical
simulations of the evolution of N2, methane and carbon monoxide
on Pluto over thousands of years. The model predicts N2 ice
accumulation in the deepest low-latitude basin and the threefold
increase in atmospheric pressure that has been observed to occur
since 19884–6. This points to atmospheric–topographic processes as
the origin of Sputnik Planum’s N2 glacier. The same simulations also
reproduce the observed quantities of volatiles in the atmosphere and
show frosts of methane, and sometimes N2, that seasonally cover the
mid- and high latitudes, explaining the bright northern polar cap
reported in the 1990s7,8 and the observed ice distribution in 20152.
The model also predicts that most of these seasonal frosts should
disappear in the next decade.
Todo mundo sabe que os raios produzidos pela Estrela da Morte em Guerra nas Estrelas não pode existir na vida real, porém no universo existem fenômenos que as vezes conseguem superar até a mais surpreendente ficção.
A galáxia Pictor A, é um desses objetos que possuem fenômenos tão espetaculares quanto aqueles exibidos no cinema. Essa galáxia localiza-se a cerca de 500 milhões de anos-luz da Terra e possui um buraco negro supermassivo no seu centro. Uma grande quantidade de energia gravitacional é lançada, à medida que o material cai em direção ao horizonte de eventos, o ponto sem volta ao redor do buraco negro. Essa energia produz um enorme jato de partículas que viajam a uma velocidade próxima da velocidade da luz no espaço intergaláctico, chamado de jato relativístico.
Para obter imagens desse jato, os cientistas usaram o Observatório de Raios-X Chandra, da NASA várias vezes durante 15 anos. Os dados do Chandra, apresentados em azul nas imagens, foram combinados com os dados obtidos em ondas de rádio a partir do Australia Telescope Compact Array, e são aparesentados em vermelho nas imagens.
A giant galaxy in the young Universe with a massive ringSérgio Sacani
In the local (redshift z ≈ 0) Universe, collisional ring galaxies make up only ~0.01% of galaxies1 and are formed by head-on galactic collisions that trigger radially propagating density waves2–4. These striking systems provide key snapshots for dissecting galactic disks and are studied extensively in the local Universe5–9. However, not much is known about distant (z > 0.1) collisional rings10–14. Here we present a detailed study of a ring galaxy at a look-back time of 10.8 Gyr (z = 2.19). Compared with our Milky Way, this galaxy has a similar stellar mass, but has a stellar half-light radius that is 1.5–2.2 times larger and is forming stars 50 times faster. The extended, dif- fuse stellar light outside the star-forming ring, combined with a radial velocity on the ring and an intruder galaxy nearby, provides evidence for this galaxy hosting a collisional ring. If the ring is secularly evolved15,16, the implied large bar in a giant disk would be inconsistent with the current understand- ing of the earliest formation of barred spirals17–21. Contrary to previous predictions10–12, this work suggests that massive col- lisional rings were as rare 11 Gyr ago as they are today. Our discovery offers a unique pathway for studying density waves in young galaxies, as well as constraining the cosmic evolution of spiral disks and galaxy groups.
Quase 900 galáxias próximas, porém escondidas, têm sido estudadas por uma equipe internacional de astrônomos, levando uma nova luz sobre o entendimento do Grande Atrator - uma concentração difusa de massa a 250 milhões de anos-luz de distância, que está puxando a nossa Via Láctea, e milhares de outras galáxias em sua direção.
Usando o Multibeam Receiver, instalado no rádio telescópio Parkes de 64 m, pertencente à instituição CSIRO na Austrália, a equipe foi capaz de ver através das estrelas e da poeira da nossa galáxia, vasculhando assim uma região inexplorada do espaço, conhecida pelos astrônomos como Zone of Avoidance (Zona de Anulação).
“Nós descobrimos 883 galáxias, um terço das quais nunca tinham sido vistas anteriormente”, disse o Professor Lister Staveley-Smith, membro da equipe, do ARC Centre of Excellence for All-sky Astrophysics, e da University of Western Australia, um dos nós do International Centre for Radio Astronomy Research.
First results from_the_hubble_opal_program_jupiter_in_2015Sérgio Sacani
Os cientistas usando o Telescópio Espacial Hubble da NASA/ESA produziram novos mapas de Júpiter, que mostram as contínuas mudanças que ocorrem com a famosa Grande Mancha Vermelha. As imagens também revelam uma rara estrutura em forma de onda na atmosfera do planeta que não tinha sido vista por décadas. A nova imagem é a primeira de uma série de retratos anuais dos planetas externos do Sistema Solar, que nos darão um novo olhar desses mundos remotos, e ajudarão os cientistas a estudarem como eles mudam com o passar do tempo.
Nessa nova imagem de Júpiter, uma grande quantidade de feições foi capturada incluindo ventos, nuvens e tempestades. Os cientistas por trás dessas novas imagens, as obtiveram usando a Wide Field Camera 3 do Hubble, num período de observação de mais de 10 horas e produziram assim dois mapas completos do planeta, a partir das suas observações. Esses mapas fizeram com que fosse possível determinar a velocidade dos ventos em Júpiter, com a finalidade de identificar diferentes fenômenos na sua atmosfera além de traquear as suas feições mais famosas.
As novas imagens confirmam que a grande tempestade que tem existido na superfície de nuvens de Júpiter por no mínimo 300 anos, continua a encolher, mas mesmo que desapareça, ela irá morrer lutando. A tempestade, conhecida como Grande Mancha Vermelha, é vista aqui fazendo seus movimentos em espiral no centro da imagem do planeta. Ela tem diminuído de tamanho de maneira muito rápida de ano em ano. Mas agora, a taxa de encolhimento parece ter reduzido novamente, mesmo apesar da mancha ser cerca de 240 quilômetros menor do que era em 2014.
Detection of lyman_alpha_emission_from_a_triply_imaged_z_6_85_galaxy_behind_m...Sérgio Sacani
We report the detection of Ly emission at 9538A
in the Keck/DEIMOS and HST WFC3
G102 grism data from a triply-imaged galaxy at z = 6:846 0:001 behind galaxy cluster MACS
J2129.4 0741. Combining the emission line wavelength with broadband photometry, line ratio upper
limits, and lens modeling, we rule out the scenario that this emission line is [O II] at z = 1:57. After
accounting for magnication, we calculate the weighted average of the intrinsic Ly luminosity to be
1:31042 erg s 1 and Ly equivalent width to be 7415A. Its intrinsic UV absolute magnitude at
1600A
is 18:60:2 mag and stellar mass (1:50:3)107 M, making it one of the faintest (intrinsic
LUV 0:14 L
UV) galaxies with Ly detection at z 7 to date. Its stellar mass is in the typical range
for the galaxies thought to dominate the reionization photon budget at z & 7; the inferred Ly escape
fraction is high (& 10%), which could be common for sub-L z & 7 galaxies with Ly emission. This
galaxy oers a glimpse of the galaxy population that is thought to drive reionization, and it shows
that gravitational lensing is an important avenue to probe the sub-L galaxy population.
A thirty-four billion solar mass black hole in SMSS J2157–3602, the most lumi...Sérgio Sacani
From near-infrared spectroscopic measurements of the Mg II emission line doublet, we estimate the black hole (BH) mass of the quasar, SMSS J215728.21–360215.1, as being (3.4 ± 0.6) × 1010 M⊙ and refine the redshift of the quasar to be z = 4.692. SMSS J2157 is the most luminous known quasar, with a 3000 Å luminosity of (4.7 ± 0.5) × 1047 erg s−1 and an estimated bolometric luminosity of 1.6 × 1048 erg s−1 , yet its Eddington ratio is only ∼0.4. Thus, the high luminosity of this quasar is a consequence of its extremely large BH – one of the most massive BHs at z > 4.
A rare case of FR I interaction with a hot X-ray bridge in the A2384 galaxy c...Sérgio Sacani
Clusters of varying mass ratios can merge and the process significantly disturbs
the cluster environments and alters their global properties. Active radio galaxies are
another phenomenon that can also affect cluster environments. Radio jets can interact
with the intra-cluster medium (ICM) and locally affect its properties. Abell 2384
(hereafter A2384) is a unique system that has a dense, hot X-ray filament or bridge
connecting the two unequal mass clusters A2384(N) and A2384(S). The analysis of its
morphology suggests that A2384 is a post-merger system where A2384(S) has already
interacted with the A2384(N), and as a result hot gas has been stripped over a ∼ 1
Mpc region between the two bodies. We have obtained its 325 MHz GMRT data,
and we detected a peculiar FR I type radio galaxy which is a part of the A2384(S).
One of its radio lobes interacts with the hot X-ray bridge and pushes the hot gas in
the opposite direction. This results in displacement in the bridge close to A2384(S).
Based on Chandra and XMM-Newton X-ray observations, we notice a temperature and
entropy enhancement at the radio lobe-X-ray plasma interaction site, which further
suggests that the radio lobe is changing thermal plasma properties. We have also
studied the radio properties of the FR I radio galaxy, and found that the size and
radio luminosity of the interacting north lobe of the FR I galaxy are lower than those
of the accompanying south lobe.
Carbon star formation as seen through the non-monotonic initial–final mass re...Sérgio Sacani
The initial–final mass relation (IFMR) links the birth mass of a star to the mass of the compact remnant left at its death. While
the relevance of the IFMR across astrophysics is universally acknowledged, not all of its fine details have yet been resolved.
A new analysis of a few carbon–oxygen white dwarfs in old open clusters of the Milky Way led us to identify a kink in the IFMR,
located over a range of initial masses, 1.65 ≲Mi
/M⊙ ≲ 2.10. The kink’s peak in white dwarf mass of about 0.70−0.75 M⊙ is
produced by stars with Mi≈ 1.8−1.9 M⊙, corresponding to ages of about 1.8−1.7 Gyr. Interestingly, this peak coincides with
the initial mass limit between low-mass stars that develop a degenerate helium core after central hydrogen exhaustion, and
intermediate-mass stars that avoid electron degeneracy. We interpret the IFMR kink as the signature of carbon star formation
in the Milky Way. This finding is critical to constraining the evolution and chemical enrichment of low-mass stars, and their
impact on the spectrophotometric properties of galaxies.
The shadow _of_the_flying_saucer_a_very_low_temperature_for_large_dust_grainsSérgio Sacani
Os astrónomos usaram o ALMA e os telescópios do IRAM para fazer a primeira medição direta da temperatura dos grãos de poeira grandes situados nas regiões periféricas de um disco de formação planetária que se encontra em torno de uma estrela jovem. Ao observar de forma inovadora um objeto cujo nome informal é Disco Voador, os astrónomos descobriram que os grãos de poeira são muito mais frios do que o esperado: -266º Celsius. Este resultado surpreendente sugere que os modelos teóricos destes discos precisam de ser revistos.
Uma equipa internacional liderada por Stephane Guilloteau do Laboratoire d´Astrophysique de Bordeaux, França, mediu a temperatura de enormes grãos de poeira que se encontram em torno da jovem estrela 2MASS J16281370-2431391 na região de formação estelar Rho Ophiuchi, a cerca de 400 anos-luz de distância da Terra.
Esta estrela encontra-se rodeada por um disco de gás e poeira — chamado disco protoplanetário, uma vez que se encontra na fase inicial da formação de um sistema planetário. Este disco é visto de perfil quando observado a partir da Terra e a sua aparência em imagens no visível levou a que se lhe desse o nome informal de Disco Voador.
Os astrónomos utilizaram o ALMA para observar o brilho emitido pelas moléculas de monóxido de carbono no disco da 2MASS J16281370-2431391. As imagens revelaram-se extremamente nítidas e descobriu-se algo estranho — em alguns casos o sinal recebido era negativo. Normalmente um sinal negativo é fisicamente impossível, mas neste caso existe uma explicação, que leva a uma conclusão surpreendente.
Detection of solar_like_oscillations_in_relies_of_the_milk_way_asteroseismolo...Sérgio Sacani
Asteroseismic constraints on K giants make it possible to infer radii, masses and ages of tens
of thousands of field stars. Tests against independent estimates of these properties are however
scarce, especially in the metal-poor regime. Here, we report the detection of solar-like
oscillations in 8 stars belonging to the red-giant branch and red-horizontal branch of the globular
cluster M4. The detections were made in photometric observations from the K2 Mission
during its Campaign 2. Making use of independent constraints on the distance, we estimate
masses of the 8 stars by utilising different combinations of seismic and non-seismic inputs.
When introducing a correction to the Δν scaling relation as suggested by stellar models, for
RGB stars we find excellent agreement with the expected masses from isochrone fitting, and
with a distance modulus derived using independent methods. The offset with respect to independent
masses is lower, or comparable with, the uncertainties on the average RGB mass
(4 − 10%, depending on the combination of constraints used). Our results lend confidence to
asteroseismic masses in the metal poor regime. We note that a larger sample will be needed
to allow more stringent tests to be made of systematic uncertainties in all the observables
(both seismic and non-seismic), and to explore the properties of RHB stars, and of different
populations in the cluster.
Observed glacier and volatile distribution on Pluto from atmosphere–topograph...Sérgio Sacani
Pluto has a variety of surface frosts and landforms as well as a
complex atmosphere1. There is ongoing geological activity related
to the massive Sputnik Planum glacier, mostly made of nitrogen (N2)
ice mixed with solid carbon monoxide and methane2, covering the
4-kilometre-deep, 1,000-kilometre-wide basin of Sputnik Planum1,3
near the anti-Charon point. The glacier has been suggested to arise
from a source region connected to the deep interior, or from a sink
collecting the volatiles released planetwide1. Thin deposits of N2
frost, however, were also detected at mid-northern latitudes and
methane ice was observed to cover most of Pluto except for the
darker, frost-free equatorial regions2. Here we report numerical
simulations of the evolution of N2, methane and carbon monoxide
on Pluto over thousands of years. The model predicts N2 ice
accumulation in the deepest low-latitude basin and the threefold
increase in atmospheric pressure that has been observed to occur
since 19884–6. This points to atmospheric–topographic processes as
the origin of Sputnik Planum’s N2 glacier. The same simulations also
reproduce the observed quantities of volatiles in the atmosphere and
show frosts of methane, and sometimes N2, that seasonally cover the
mid- and high latitudes, explaining the bright northern polar cap
reported in the 1990s7,8 and the observed ice distribution in 20152.
The model also predicts that most of these seasonal frosts should
disappear in the next decade.
Todo mundo sabe que os raios produzidos pela Estrela da Morte em Guerra nas Estrelas não pode existir na vida real, porém no universo existem fenômenos que as vezes conseguem superar até a mais surpreendente ficção.
A galáxia Pictor A, é um desses objetos que possuem fenômenos tão espetaculares quanto aqueles exibidos no cinema. Essa galáxia localiza-se a cerca de 500 milhões de anos-luz da Terra e possui um buraco negro supermassivo no seu centro. Uma grande quantidade de energia gravitacional é lançada, à medida que o material cai em direção ao horizonte de eventos, o ponto sem volta ao redor do buraco negro. Essa energia produz um enorme jato de partículas que viajam a uma velocidade próxima da velocidade da luz no espaço intergaláctico, chamado de jato relativístico.
Para obter imagens desse jato, os cientistas usaram o Observatório de Raios-X Chandra, da NASA várias vezes durante 15 anos. Os dados do Chandra, apresentados em azul nas imagens, foram combinados com os dados obtidos em ondas de rádio a partir do Australia Telescope Compact Array, e são aparesentados em vermelho nas imagens.
A giant galaxy in the young Universe with a massive ringSérgio Sacani
In the local (redshift z ≈ 0) Universe, collisional ring galaxies make up only ~0.01% of galaxies1 and are formed by head-on galactic collisions that trigger radially propagating density waves2–4. These striking systems provide key snapshots for dissecting galactic disks and are studied extensively in the local Universe5–9. However, not much is known about distant (z > 0.1) collisional rings10–14. Here we present a detailed study of a ring galaxy at a look-back time of 10.8 Gyr (z = 2.19). Compared with our Milky Way, this galaxy has a similar stellar mass, but has a stellar half-light radius that is 1.5–2.2 times larger and is forming stars 50 times faster. The extended, dif- fuse stellar light outside the star-forming ring, combined with a radial velocity on the ring and an intruder galaxy nearby, provides evidence for this galaxy hosting a collisional ring. If the ring is secularly evolved15,16, the implied large bar in a giant disk would be inconsistent with the current understand- ing of the earliest formation of barred spirals17–21. Contrary to previous predictions10–12, this work suggests that massive col- lisional rings were as rare 11 Gyr ago as they are today. Our discovery offers a unique pathway for studying density waves in young galaxies, as well as constraining the cosmic evolution of spiral disks and galaxy groups.
Quase 900 galáxias próximas, porém escondidas, têm sido estudadas por uma equipe internacional de astrônomos, levando uma nova luz sobre o entendimento do Grande Atrator - uma concentração difusa de massa a 250 milhões de anos-luz de distância, que está puxando a nossa Via Láctea, e milhares de outras galáxias em sua direção.
Usando o Multibeam Receiver, instalado no rádio telescópio Parkes de 64 m, pertencente à instituição CSIRO na Austrália, a equipe foi capaz de ver através das estrelas e da poeira da nossa galáxia, vasculhando assim uma região inexplorada do espaço, conhecida pelos astrônomos como Zone of Avoidance (Zona de Anulação).
“Nós descobrimos 883 galáxias, um terço das quais nunca tinham sido vistas anteriormente”, disse o Professor Lister Staveley-Smith, membro da equipe, do ARC Centre of Excellence for All-sky Astrophysics, e da University of Western Australia, um dos nós do International Centre for Radio Astronomy Research.
First results from_the_hubble_opal_program_jupiter_in_2015Sérgio Sacani
Os cientistas usando o Telescópio Espacial Hubble da NASA/ESA produziram novos mapas de Júpiter, que mostram as contínuas mudanças que ocorrem com a famosa Grande Mancha Vermelha. As imagens também revelam uma rara estrutura em forma de onda na atmosfera do planeta que não tinha sido vista por décadas. A nova imagem é a primeira de uma série de retratos anuais dos planetas externos do Sistema Solar, que nos darão um novo olhar desses mundos remotos, e ajudarão os cientistas a estudarem como eles mudam com o passar do tempo.
Nessa nova imagem de Júpiter, uma grande quantidade de feições foi capturada incluindo ventos, nuvens e tempestades. Os cientistas por trás dessas novas imagens, as obtiveram usando a Wide Field Camera 3 do Hubble, num período de observação de mais de 10 horas e produziram assim dois mapas completos do planeta, a partir das suas observações. Esses mapas fizeram com que fosse possível determinar a velocidade dos ventos em Júpiter, com a finalidade de identificar diferentes fenômenos na sua atmosfera além de traquear as suas feições mais famosas.
As novas imagens confirmam que a grande tempestade que tem existido na superfície de nuvens de Júpiter por no mínimo 300 anos, continua a encolher, mas mesmo que desapareça, ela irá morrer lutando. A tempestade, conhecida como Grande Mancha Vermelha, é vista aqui fazendo seus movimentos em espiral no centro da imagem do planeta. Ela tem diminuído de tamanho de maneira muito rápida de ano em ano. Mas agora, a taxa de encolhimento parece ter reduzido novamente, mesmo apesar da mancha ser cerca de 240 quilômetros menor do que era em 2014.
Detection of lyman_alpha_emission_from_a_triply_imaged_z_6_85_galaxy_behind_m...Sérgio Sacani
We report the detection of Ly emission at 9538A
in the Keck/DEIMOS and HST WFC3
G102 grism data from a triply-imaged galaxy at z = 6:846 0:001 behind galaxy cluster MACS
J2129.4 0741. Combining the emission line wavelength with broadband photometry, line ratio upper
limits, and lens modeling, we rule out the scenario that this emission line is [O II] at z = 1:57. After
accounting for magnication, we calculate the weighted average of the intrinsic Ly luminosity to be
1:31042 erg s 1 and Ly equivalent width to be 7415A. Its intrinsic UV absolute magnitude at
1600A
is 18:60:2 mag and stellar mass (1:50:3)107 M, making it one of the faintest (intrinsic
LUV 0:14 L
UV) galaxies with Ly detection at z 7 to date. Its stellar mass is in the typical range
for the galaxies thought to dominate the reionization photon budget at z & 7; the inferred Ly escape
fraction is high (& 10%), which could be common for sub-L z & 7 galaxies with Ly emission. This
galaxy oers a glimpse of the galaxy population that is thought to drive reionization, and it shows
that gravitational lensing is an important avenue to probe the sub-L galaxy population.
A thirty-four billion solar mass black hole in SMSS J2157–3602, the most lumi...Sérgio Sacani
From near-infrared spectroscopic measurements of the Mg II emission line doublet, we estimate the black hole (BH) mass of the quasar, SMSS J215728.21–360215.1, as being (3.4 ± 0.6) × 1010 M⊙ and refine the redshift of the quasar to be z = 4.692. SMSS J2157 is the most luminous known quasar, with a 3000 Å luminosity of (4.7 ± 0.5) × 1047 erg s−1 and an estimated bolometric luminosity of 1.6 × 1048 erg s−1 , yet its Eddington ratio is only ∼0.4. Thus, the high luminosity of this quasar is a consequence of its extremely large BH – one of the most massive BHs at z > 4.
A rare case of FR I interaction with a hot X-ray bridge in the A2384 galaxy c...Sérgio Sacani
Clusters of varying mass ratios can merge and the process significantly disturbs
the cluster environments and alters their global properties. Active radio galaxies are
another phenomenon that can also affect cluster environments. Radio jets can interact
with the intra-cluster medium (ICM) and locally affect its properties. Abell 2384
(hereafter A2384) is a unique system that has a dense, hot X-ray filament or bridge
connecting the two unequal mass clusters A2384(N) and A2384(S). The analysis of its
morphology suggests that A2384 is a post-merger system where A2384(S) has already
interacted with the A2384(N), and as a result hot gas has been stripped over a ∼ 1
Mpc region between the two bodies. We have obtained its 325 MHz GMRT data,
and we detected a peculiar FR I type radio galaxy which is a part of the A2384(S).
One of its radio lobes interacts with the hot X-ray bridge and pushes the hot gas in
the opposite direction. This results in displacement in the bridge close to A2384(S).
Based on Chandra and XMM-Newton X-ray observations, we notice a temperature and
entropy enhancement at the radio lobe-X-ray plasma interaction site, which further
suggests that the radio lobe is changing thermal plasma properties. We have also
studied the radio properties of the FR I radio galaxy, and found that the size and
radio luminosity of the interacting north lobe of the FR I galaxy are lower than those
of the accompanying south lobe.
Carbon star formation as seen through the non-monotonic initial–final mass re...Sérgio Sacani
The initial–final mass relation (IFMR) links the birth mass of a star to the mass of the compact remnant left at its death. While
the relevance of the IFMR across astrophysics is universally acknowledged, not all of its fine details have yet been resolved.
A new analysis of a few carbon–oxygen white dwarfs in old open clusters of the Milky Way led us to identify a kink in the IFMR,
located over a range of initial masses, 1.65 ≲Mi
/M⊙ ≲ 2.10. The kink’s peak in white dwarf mass of about 0.70−0.75 M⊙ is
produced by stars with Mi≈ 1.8−1.9 M⊙, corresponding to ages of about 1.8−1.7 Gyr. Interestingly, this peak coincides with
the initial mass limit between low-mass stars that develop a degenerate helium core after central hydrogen exhaustion, and
intermediate-mass stars that avoid electron degeneracy. We interpret the IFMR kink as the signature of carbon star formation
in the Milky Way. This finding is critical to constraining the evolution and chemical enrichment of low-mass stars, and their
impact on the spectrophotometric properties of galaxies.
Jonathan Lee, Managing Director, Brand Strategy, and Ken Allard, Managing Director, Business Strategy at HUGE, gave this presentation at "Ambidexterity 2," the VCU Brandcenter's Executive Education program for account planning on June 24th at the VCU Brandcenter in Richmond, VA.
On some structural_features_of_the_metagalaxySérgio Sacani
Progress in a group of investigations designed
to discover some of the structural details in individual galaxies and in the
Metagalaxy is reported in the following pages.
(a) The first section is concerned with the distribution of cluster-type
Cepheids in high galactic latitude. To the 169 already known in latitudes,
greater than or equal to ± 20o
, the systematic variable star programme carried
on at Harvard has added 312, mostly fainter than magnitude 13-0. With
allowance for absorption and for uncertainties yet remaining in the mean
absolute magnitude of these stars, the thickness of the Milky Way, so far
as this type of star is concerned, is not less than twenty-five kiloparsecs ;
he extent of the Milky Way in its own plane, by the same criterion, is more
than thirty kiloparsecs, perhaps much more.
(b) The extent of the Milky Way in the anti-centre quadrant is considered
on the basis of classical and cluster-type Cepheids ; provisionally
it is found that the galactic system reaches to a distance of at least ten
kiloparsecs in longitude 150o
.
(r) More than six hundred new variables have been found in the Large
Magellanic Cloud and measured for position, ranges and median magnitudes ;
the frequency of periods is not unlike that for the classical Cepheids in the
galactic system ; the light curves also are comparable in all details. The
Magellanic Cepheids, like the galactic classical Cepheids, are concentrated
in regions of high star-density.
(d) Further study of the period-luminosity relation in the Large Magellanic
Cloud permits its revision and strengthening for the Cepheids of
highest absolute magnitude. An observed deviation from the relation
that had previously been found for the Small Cloud is probably to be
attributed to scale error in the magnitude system. No seriously disturbing
We report the discovery of spiral galaxies that are as optically luminous as elliptical brightest cluster
galaxies, with r-band monochromatic luminosity Lr = 8 14L (4:3 7:5 1044 erg s 1). These
super spiral galaxies are also giant and massive, with diameter D = 57 134 kpc and stellar mass
Mstars = 0:3 3:4 1011M. We nd 53 super spirals out of a complete sample of 1616 SDSS
galaxies with redshift z < 0:3 and Lr > 8L. The closest example is found at z = 0:089. We use
existing photometry to estimate their stellar masses and star formation rates (SFRs). The SDSS
and WISE colors are consistent with normal star-forming spirals on the blue sequence. However, the
extreme masses and rapid SFRs of 5 65M yr 1 place super spirals in a sparsely populated region
of parameter space, above the star-forming main sequence of disk galaxies. Super spirals occupy a
diverse range of environments, from isolation to cluster centers. We nd four super spiral galaxy
systems that are late-stage major mergers{a possible clue to their formation. We suggest that super
spirals are a remnant population of unquenched, massive disk galaxies. They may eventually become
massive lenticular galaxies after they are cut o from their gas supply and their disks fade.
A 17 billion_solar_mass_black_hole_in_a_group_galaxy_with_a_difuse_coreSérgio Sacani
Quasars are associated with and powered by the accretion of
material onto massive black holes; the detection of highly luminous
quasars with redshifts greater than z = 6 suggests that black holes of
up to ten billion solar masses already existed 13 billion years ago1.
Two possible present-day ‘dormant’ descendants of this population
of ‘active’ black holes have been found2 in the galaxies NGC 3842
and NGC 4889 at the centres of the Leo and Coma galaxy clusters,
which together form the central region of the Great Wall3—the
largest local structure of galaxies. The most luminous quasars,
however, are not confined to such high-density regions of the
early Universe4,5; yet dormant black holes of this high mass have
not yet been found outside of modern-day rich clusters. Here we
report observations of the stellar velocity distribution in the galaxy
NGC 1600—a relatively isolated elliptical galaxy near the centre
of a galaxy group at a distance of 64 megaparsecs from Earth. We
use orbit superposition models to determine that the black hole at
the centre of NGC 1600 has a mass of 17 billion solar masses. The
spatial distribution of stars near the centre of NGC 1600 is rather
diffuse. We find that the region of depleted stellar density in the
cores of massive elliptical galaxies extends over the same radius as
the gravitational sphere of influence of the central black holes, and
interpret this as the dynamical imprint of the black holes.
An almost dark galaxy with the mass of the Small Magellanic CloudSérgio Sacani
Almost Dark Galaxies are objects that have eluded detection by traditional surveys such as the Sloan Digital Sky Survey (SDSS). The
low surface brightness of these galaxies (µr(0)> 26 mag/arcsec2
), and hence their low surface stellar mass density (a few solar masses
per pc2 or less), suggests that the energy density released by baryonic feedback mechanisms is inefficient in modifying the distribution
of the dark matter halos they inhabit. For this reason, almost dark galaxies are particularly promising for probing the microphysical
nature of dark matter. In this paper, we present the serendipitous discovery of Nube, an almost dark galaxy with < µV >e∼ 26.7
mag/arcsec2
. The galaxy was identified using deep optical imaging from the IAC Stripe82 Legacy Project. Follow-up observations
with the 100m Green Bank Telescope strongly suggest that the galaxy is at a distance of 107 Mpc. Ultra-deep multi-band observations
with the 10.4m Gran Telescopio Canarias favour an age of ∼ 10 Gyr and a metallicity of [Fe/H]∼ −1.1. With a stellar mass of ∼ 4×108
M⊙ and a half-mass radius of Re = 6.9 kpc (corresponding to an effective surface density of < Σ >e∼ 0.9 M⊙/pc2
), Nube is the most
massive and extended object of its kind discovered so far. The galaxy is ten times fainter and has an effective radius three times larger
than typical ultra-diffuse galaxies with similar stellar masses. Galaxies with comparable effective surface brightness within the Local
Group have very low mass (tens of 105 M⊙) and compact structures (effective radius Re < 1 kpc). Current cosmological simulations
within the cold dark matter scenario, including baryonic feedback, do not reproduce the structural properties of Nube. However, its
highly extended and flattened structure is consistent with a scenario where the dark matter particles are ultra-light axions with a mass
of mB=(0.8
+0.4
−0.2
)×10−23 eV
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.
Large scale mass_distribution_in_the_illustris_simulationSérgio Sacani
Observations at low redshifts thus far fail to account for all of the baryons expected in the
Universe according to cosmological constraints. A large fraction of the baryons presumably
resides in a thin and warm–hot medium between the galaxies, where they are difficult to observe
due to their low densities and high temperatures. Cosmological simulations of structure
formation can be used to verify this picture and provide quantitative predictions for the distribution
of mass in different large-scale structure components. Here we study the distribution
of baryons and dark matter at different epochs using data from the Illustris simulation. We
identify regions of different dark matter density with the primary constituents of large-scale
structure, allowing us to measure mass and volume of haloes, filaments and voids. At redshift
zero, we find that 49 per cent of the dark matter and 23 per cent of the baryons are within
haloes more massive than the resolution limit of 2 × 108 M⊙. The filaments of the cosmic
web host a further 45 per cent of the dark matter and 46 per cent of the baryons. The remaining
31 per cent of the baryons reside in voids. The majority of these baryons have been transported
there through active galactic nuclei feedback. We note that the feedback model of Illustris
is too strong for heavy haloes, therefore it is likely that we are overestimating this amount.
Categorizing the baryons according to their density and temperature, we find that 17.8 per cent
of them are in a condensed state, 21.6 per cent are present as cold, diffuse gas, and 53.9 per cent
are found in the state of a warm–hot intergalactic medium.
Similar to Galaxy dynamics and the mass density of the universe (20)
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.
WASP-69b’s Escaping Envelope Is Confined to a Tail Extending at Least 7 RpSérgio Sacani
Studying the escaping atmospheres of highly irradiated exoplanets is critical for understanding the physical
mechanisms that shape the demographics of close-in planets. A number of planetary outflows have been observed
as excess H/He absorption during/after transit. Such an outflow has been observed for WASP-69b by multiple
groups that disagree on the geometry and velocity structure of the outflow. Here, we report the detection of this
planet’s outflow using Keck/NIRSPEC for the first time. We observed the outflow 1.28 hr after egress until the
target set, demonstrating the outflow extends at least 5.8 × 105 km or 7.5 Rp This detection is significantly longer
than previous observations, which report an outflow extending ∼2.2 planet radii just 1 yr prior. The outflow is
blueshifted by −23 km s−1 in the planetary rest frame. We estimate a current mass-loss rate of 1 M⊕ Gyr−1
. Our
observations are most consistent with an outflow that is strongly sculpted by ram pressure from the stellar wind.
However, potential variability in the outflow could be due to time-varying interactions with the stellar wind or
differences in instrumental precision.
X-rays from a Central “Exhaust Vent” of the Galactic Center ChimneySérgio Sacani
Using deep archival observations from the Chandra X-ray Observatory, we present an analysis of
linear X-ray-emitting features located within the southern portion of the Galactic center chimney,
and oriented orthogonal to the Galactic plane, centered at coordinates l = 0.08◦
, b = −1.42◦
. The
surface brightness and hardness ratio patterns are suggestive of a cylindrical morphology which may
have been produced by a plasma outflow channel extending from the Galactic center. Our fits of the
feature’s spectra favor a complex two-component model consisting of thermal and recombining plasma
components, possibly a sign of shock compression or heating of the interstellar medium by outflowing
material. Assuming a recombining plasma scenario, we further estimate the cooling timescale of this
plasma to be on the order of a few hundred to thousands of years, leading us to speculate that a
sequence of accretion events onto the Galactic Black Hole may be a plausible quasi-continuous energy
source to sustain the observed morphology
Cancer cell metabolism: special Reference to Lactate PathwayAADYARAJPANDEY1
Normal Cell Metabolism:
Cellular respiration describes the series of steps that cells use to break down sugar and other chemicals to get the energy we need to function.
Energy is stored in the bonds of glucose and when glucose is broken down, much of that energy is released.
Cell utilize energy in the form of ATP.
The first step of respiration is called glycolysis. In a series of steps, glycolysis breaks glucose into two smaller molecules - a chemical called pyruvate. A small amount of ATP is formed during this process.
Most healthy cells continue the breakdown in a second process, called the Kreb's cycle. The Kreb's cycle allows cells to “burn” the pyruvates made in glycolysis to get more ATP.
The last step in the breakdown of glucose is called oxidative phosphorylation (Ox-Phos).
It takes place in specialized cell structures called mitochondria. This process produces a large amount of ATP. Importantly, cells need oxygen to complete oxidative phosphorylation.
If a cell completes only glycolysis, only 2 molecules of ATP are made per glucose. However, if the cell completes the entire respiration process (glycolysis - Kreb's - oxidative phosphorylation), about 36 molecules of ATP are created, giving it much more energy to use.
IN CANCER CELL:
Unlike healthy cells that "burn" the entire molecule of sugar to capture a large amount of energy as ATP, cancer cells are wasteful.
Cancer cells only partially break down sugar molecules. They overuse the first step of respiration, glycolysis. They frequently do not complete the second step, oxidative phosphorylation.
This results in only 2 molecules of ATP per each glucose molecule instead of the 36 or so ATPs healthy cells gain. As a result, cancer cells need to use a lot more sugar molecules to get enough energy to survive.
Unlike healthy cells that "burn" the entire molecule of sugar to capture a large amount of energy as ATP, cancer cells are wasteful.
Cancer cells only partially break down sugar molecules. They overuse the first step of respiration, glycolysis. They frequently do not complete the second step, oxidative phosphorylation.
This results in only 2 molecules of ATP per each glucose molecule instead of the 36 or so ATPs healthy cells gain. As a result, cancer cells need to use a lot more sugar molecules to get enough energy to survive.
introduction to WARBERG PHENOMENA:
WARBURG EFFECT Usually, cancer cells are highly glycolytic (glucose addiction) and take up more glucose than do normal cells from outside.
Otto Heinrich Warburg (; 8 October 1883 – 1 August 1970) In 1931 was awarded the Nobel Prize in Physiology for his "discovery of the nature and mode of action of the respiratory enzyme.
WARNBURG EFFECT : cancer cells under aerobic (well-oxygenated) conditions to metabolize glucose to lactate (aerobic glycolysis) is known as the Warburg effect. Warburg made the observation that tumor slices consume glucose and secrete lactate at a higher rate than normal tissues.
(May 29th, 2024) Advancements in Intravital Microscopy- Insights for Preclini...Scintica Instrumentation
Intravital microscopy (IVM) is a powerful tool utilized to study cellular behavior over time and space in vivo. Much of our understanding of cell biology has been accomplished using various in vitro and ex vivo methods; however, these studies do not necessarily reflect the natural dynamics of biological processes. Unlike traditional cell culture or fixed tissue imaging, IVM allows for the ultra-fast high-resolution imaging of cellular processes over time and space and were studied in its natural environment. Real-time visualization of biological processes in the context of an intact organism helps maintain physiological relevance and provide insights into the progression of disease, response to treatments or developmental processes.
In this webinar we give an overview of advanced applications of the IVM system in preclinical research. IVIM technology is a provider of all-in-one intravital microscopy systems and solutions optimized for in vivo imaging of live animal models at sub-micron resolution. The system’s unique features and user-friendly software enables researchers to probe fast dynamic biological processes such as immune cell tracking, cell-cell interaction as well as vascularization and tumor metastasis with exceptional detail. This webinar will also give an overview of IVM being utilized in drug development, offering a view into the intricate interaction between drugs/nanoparticles and tissues in vivo and allows for the evaluation of therapeutic intervention in a variety of tissues and organs. This interdisciplinary collaboration continues to drive the advancements of novel therapeutic strategies.
Seminar of U.V. Spectroscopy by SAMIR PANDASAMIR PANDA
Spectroscopy is a branch of science dealing the study of interaction of electromagnetic radiation with matter.
Ultraviolet-visible spectroscopy refers to absorption spectroscopy or reflect spectroscopy in the UV-VIS spectral region.
Ultraviolet-visible spectroscopy is an analytical method that can measure the amount of light received by the analyte.
Nutraceutical market, scope and growth: Herbal drug technologyLokesh Patil
As consumer awareness of health and wellness rises, the nutraceutical market—which includes goods like functional meals, drinks, and dietary supplements that provide health advantages beyond basic nutrition—is growing significantly. As healthcare expenses rise, the population ages, and people want natural and preventative health solutions more and more, this industry is increasing quickly. Further driving market expansion are product formulation innovations and the use of cutting-edge technology for customized nutrition. With its worldwide reach, the nutraceutical industry is expected to keep growing and provide significant chances for research and investment in a number of categories, including vitamins, minerals, probiotics, and herbal supplements.
Comparative structure of adrenal gland in vertebrates
Galaxy dynamics and the mass density of the universe
1. Proc. Natl. Acad. Sci. USA
Vol. 90, pp. 4814-4821, June 1993
Colloquium Paper
This paper was presented at a colloquium entitled "Physical Cosmology," organized by a committee chaired by David
N. Schramm, held March 27 and 28, 1992, at the National Academy of Sciences, Irvine, CA.
Galaxy dynamics and the mass density of the universe
VERA C. RUBIN
Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015
ABSTRACT Dynamical evidence accumulated over the
past 20 years has convinced astronomers that luminous matter
in a spiral galaxy constitutes no more than 10% of the mass of
a galaxy. An additional 90% is inferred by its gravitational
effect on luminous material. Here I review recent observations
concerning the distribution of luminous and nonluminous
matter in the Milky Way, in galaxies, and in galaxy clusters.
Observations of neutral hydrogen disks, some extending in
radius several times the optical disk, confirm that a massive
dark halo is a major component of virtually every spiral. A
recent surprise has been the discovery that stellar and gas
motions in ellipticals are enormously complex. To date, only for
a few spheroidal galaxies do the velocities extend far enough to
probe the outer mass distribution. But the diverse kinematics
of inner cores, peripheral to deducing the overall mass distri-
bution, offer additional evidence that ellipticals have acquired
gas-rich systems after initial formation. Dynamical results are
consistent with a low-density universe, in which the required
dark matter could be baryonic. On smallest scales of galaxies
[10 kiloparsec (kpc); H. = 50 kmsec'lmegaparsec'11 the
luminous matter constitutes only 1% ofthe closure density. On
scales greater than binary galaxies (i.e., .100 kpc) all systems
indicate a density -10% of the closure density, a density
consistent with the low baryon density in the universe. If
large-scale motions in the universe require a higher mass
density, these motions would constitute the first dynamical
evidence for nonbaryonic matter in a universe of higher
density.
Twenty years ago, astronomers started accumulating obser-
vations that indicated that orbital velocities of stars and gas
in spiral galaxies remained high out to the limits of the
observable galaxy. By 1982, after a decade ofinitial disquiet,
most astronomers reluctantly accepted the conclusion that a
galaxy consists of much more than the luminous stars, gas,
and dust that can be observed at various wavelengths. In fact,
at least 90% of a galaxy consists of dark matter, matter that
is undetectable at any wavelength and that is deduced only by
its gravitational influence on the orbits ofthe stars and the gas
that we can see. But not all astronomers were reluctant to
accept the evidence. "There are reasons, increasing in num-
ber and quality, to believe that the masses of ordinary
galaxies may have been underestimated by a factor of 10 or
more," stated Ostriker, Peebles, and Yahill (1) in a truly
prescient sentence.
This revolution in our understanding came about because
theoretical arguments revealed the necessity ofstabilizing the
disks of galaxies with invisible halos (2), because observa-
tions with optical (3, 4) and with radio (5) telescopes showed
uniformly high rotation velocities, and because an enor-
mously influential review of the inferred distribution of
The publication costs ofthis article were defrayed in part by page charge
payment. This article must therefore be hereby marked "advertisement"
in accordance with 18 U.S.C. §1734 solely to indicate this fact.
matter in galaxies (6) showed the validity of a universe in
which most ofthe matter is dark. In the decade ofthe 1980s,
observational and theoretical investigations of the dark mat-
ter became one of the most active fields of extragalactic
astronomy, and excellent reviews exist (7, 8).
For this paper, I describe a handful of recent results that I
consider to be among the most significant of the last few
years. They relate to the dynamics of disk and elliptical
galaxies, to the distribution ofnonluminous matterin galaxies
and in clusters, and to estimates of the overall mass density
of the universe.
Section I. The Milky Way and the Local Group Are Massive
We live in a large, massive spiral galaxy, the Milky Way, with
a nearby equally large and massive companion spiral, the
Andromeda galaxy. Together, these contain most ofthe mass
within the Local Group. For the past several decades,
determinations of the mass within our galaxy and the Local
Group have been imprecise. This was due principally to the
lack ofaccurate velocities and lack ofknowledge oforbits for
the outlying globular clusters and small satellite galaxies
whose motions are used to map the gravitational potential of
the Galaxy, and a lack of knowledge of the evolutionary
history ofthe dwarfsatellites. Recently, more accurate radial
velocities and distances of 47 objects on the outskirts of our
galaxy have become available. Based on the assumptions that
(i) the distribution of dark and luminous matter can be
modeled by a power-law function, and (ii) that the velocity
distribution of the outer Galaxy is isotropic, Kulessa and
Lynden-Bell (9) estimate a Galaxy mass of 1.3 x 1012 solar
mass units (MO) out to the distance sampled by the test
objects, 230 kiloparsec (kpc; 1 pc = 3.09 x 1016 m). Based on
the earlier Schmidt (10) model ofthe Galaxy, a mass of =1.9
x 1011 Mo is located within 25 kpc ofthe center (11). Hence,
the new analysis increases the radius, the mass, and the
mass-to-light ratio (M/LB in solar units, with the luminosity
measured in the blue spectral band) each by nearly an order
of magnitude. At the 25-kpc limit of the optical disk, M/LB
10, and the dark matter constitutes 70% of the total mass
(11); at 230 kpc, M/Lg 50, and dark matter constitutes
>90%o of the mass (9).
A statistical analysis using the velocities of satellites or-
biting a homogeneous set of spiral galaxies similar to the
Milky Way (12) leads to a mass of order 2 x 1012 MO within
200 kpc, in agreement with that discussed above. This large
mass and mass-to-luminosity ratio for our galaxy (and hence
also for Galaxy M31) is also in comfortable agreement with
a mass of -6.4 x 1012 Me determined by Peebles (13) for the
Local Group. Note that a radius of 230 kpc reaches suspi-
ciously close to the 350-kpc half-way point between us and
the Andromeda Galaxy. If the dark halos of our Galaxy and
Abbreviations: MIL, mass-to-light ratio; HI disk, neutral hydrogen
disk; pc, parsec; M<>, solar mass unit.
4814
2. Proc. Natl. Acad. Sci. USA 90 (1993) 4815
M31 reach each other, the values of MILB will increase to
nearly 100, a value of MIL similar to that for other systems
of similar radial scale. I discuss MIL values as a function of
radius in Section IV below.
Section II. AB Spiral Galaxies Need Dark Matter (Halos)
For spirals more distant than M31, velocity information is
usually obtained from spectral observations of ionized hy-
drogen (Ha) sampled by a long-slit spectrograph aligned
along the major axis and from 21-cm mapping ofvelocities of
the neutral hydrogen gas. To derive a mass distribution from
a velocity distribution, it is generally assumed that Newto-
nian gravitational theory is valid, that a self-gravitating
luminous disk of constant MIL dominates at small radial
distances, that a dark isothermal halo dominates at large
radial distances, and that the internal extinction is not a
function of radius. Surely the internal extinction merits a
more sophisticated treatment, as does the distribution of
mass in the halo, but the above simplifications suffice at
present.
Outermost Regions. Virtually all observed spiral rotation
curves are flat (or rising) to the limits of the observations,
implying that the mass density decreases approximately as
1/r2. The luminosity however falls off faster, exponentially.
Thus, the ratio of mass density to luminosity density in-
creases with radial distance, requiring the presence of a halo
of dark matter, which becomes increasingly dominant with
increased nuclear distance. A major requirement for deriving
meaningful parameters for the halo and for the disk is that
rotation velocities extend from the nucleus to at least several
disk scale lengths (where one scale length equals the radial
distance at which the disk surface brightness has fallen by
lle). Velocity measures for spirals now extend this far,
especially for galaxies whose neutral hydrogen disks (HI
disks) are several times more extended than their optical
disks. I show in Fig. 1 an optical image of the M101 Galaxy,
with the superposed velocity contours of the neutral hydro-
15°50'-
a
0)
v-
(301
a
1520'I-
gen disk; an HI disk with a radius three or four times that of
the optical disk is quite impressive.
To determine the contribution to the rotational velocities
from the mass of the halo, the contribution to the velocity of
a constant M/L disk must first be subtracted in quadrature
from the observed velocities. However, even an extended
rotation curve is insufficient to constrain the mass distribu-
tion because MIL of the disk is diffi'cult to pin down. For
example, velocities in the NGC 3198 Galaxy can be well-
fitted by models with a disk ofa range ofmasses, from a disk
of maximum mass (MIL = 4.4) down to a disk of zero mass
(ref. 15; Fig. 2). During the mid-1980s, it was customary to
analyze rotation curves by adopting the maximum mass disk
consistent with the inner rise ofthe rotation curve (17). These
models generally resulted in dark and luminous mass ofabout
equal amounts within the optical galaxy, but it must be
remembered that the halo mass so determined is only a lower
limit.
A major conceptual advance came when Athanassoula and
colleagues (18) pointed out that information concerning the
spiral structure of the disk could be used as an additional
constraint on the relative halo-to-disk mass. Forgalaxies with
low mass halos, the disk is unstable against the formation of
a nonaxisymmetric m = 1 mode. Adding mass to the halo
inhibits the m = 1 mode, until the m = 2 (i.e., two-armed)
mode dominates. By requiring m = 2 (but no m = 3) modes,
disk masses of order one-half the maximum disk mass
generally result. Even more importantly, these authors dem-
onstrated that every spiral with a rotation curve extending
several disk scale lengths requires dark matter, finally putting
to rest the belief (19) that some galaxies (always only those
with not very extended rotation curves) could be modeled
with no dark halos.
In an observational survey of spirals with extended HI
disks, Casertano and van Gorkom (20) discovered two spiral
galaxies, NGC 2683 and 3521, whose extended HI disks
exhibit falling velocities beyond the optical disk. Surpris-
ingly, dark matter is required even forthese spirals. ForNGC
(194m
R.A. (1950)
1h33m
FIG. 1. An optical image of the spiral galaxy M101 with velocities (V) from 21-cm neutral hydrogen (HI) superposed (14). The hydrogen
diameter is about four times the optical diameter. Angular coordinates on the sky (right ascension and declination) are the indicated coordinates.
401-
30'[-
0
r
Colloquium Paper: Rubin
i I
3. Proc. Natl. Acad. Sci. USA 90 (1993)
P I
A
a - 8-5 kpc
r 2
p0 0-0075 Mo pc-3
M 4-4
Lv,disk
a - 1-3kpc
y - 205
po - 0-27 M, pc-3
M
1.3
Lv, disk B
60
-S1-5 kpc
N t s~~~~~~~~~~2-25
50 - 0 33 Mo p-3
_L__ d i s k
0 10 20 30 40 50
r, kpc
FIG. 2. Three alternative decompositions ofthe rotation curve of
NGC 3198 (15). The observed velocities (16) are shown as points,
with the model disk and halo-rotation curves plus their sum in
quadrature (curves). The model parameters are given at right, for the
assumed halo-density distribution p. Note the large ranges permitted
by the models for values of central densities po and disk MIL values.
2683, even the maximum acceptable disk (MIL = 8, set
mostly by the maximum of the velocities) predicts velocities
falling more rapidly than observed (Fig. 3). The authors argue
that there is also a lower limit to the disk mass, for too much
halo mass produces too flat a rotation curve. It is unclear how
much this conclusion would be altered if the innermost
optical velocities, discussed in Innermost regions below, had
been available. These near-nuclear velocities are not observ-
able at the low spatial resolution of the 21-cm observations.
From these and many other recent observations of veloc-
ities in the outermost regions of spirals, I draw the following
conclusions. Virtually all spirals need dark matter in amounts
significantly greater than the luminous matter in order to
reproduce the observed velocities. Beyond the optical gal-
axy, rotation velocities in the gas disk are generally constant,
implying masses that continue to rise linearly with radius,
even in those regions where the optical luminosity is essen-
tially nil. Hence, local values of MIL are enormous. Conse-
quently, only lower limits to the mass and to MIL values are
available for spiral galaxies. The only exceptions to this
generalization are tidally distorted galaxies, and a few mod-
erately peculiar spirals in dense regions near the cores of
galaxy clusters (21), whose velocities decrease with in-
creased nuclear distances.
Innermost Regions. Advances in radio astronomy tech-
niques have permitted the detection and velocity mapping of
faint outer disks and, consequently, the determination of the
mass interior to large radial distances. Corresponding ad-
vances in optical instrumentation have permitted the spectral
40
ua0)
2a
u
0
._
to
Ca
0
100 50
C
-~~ 4
0
d
v) r SNGC 2683 MAJOR
0 Ha
NE 0 [NIl]
135 270 405
Distance from nucleus on sky, arcsec
540
FIG. 3. (A) NGC 2683, an Sb spiral, from a Kitt Peak 36-inch
charge-coupled device image. Below it, a portion of a Kitt Peak 4-m
spectrum, showing Ha (Upper) and [NII] emission. Note the mul-
tivalued emission in the inner regions. (B) The measured optical
velocities in the inner region from the spectrum in A, quantifying the
rapid rise across the nucleus and the multivalued velocities. The
surface brightness profile of the galaxy is shown by the fine line. (C)
HI velocities in NGC 2683 (20). The solid line represents the
predicted velocities based on a model with no dark matter. Thus,
even the falling rotation velocities are higher than the velocities
predicted by the light. Note the change in radial scale from B to C;
all the velocities in B lie between r =
0 and the first two 21-cm points.
observations of inner-core regions at high spatial and high
velocity resolution. Moreover, the use of digital detectors
(charge-coupled devices) allows the smooth galaxy stellar
continuum, strong in the inner regions, to be easily removed
from the spectrum, thus revealing the emission ofthe ionized
4816 Colloquium Paper: Rubin
-50 -100
4. Proc. Natl. Acad. Sci. USA 90 (1993) 4817
gas at enhanced signal-to-noise ratio. From these observa-
tions we now know that gas in central regions often exhibits
complex velocity patterns arising from a variety of causes:
warps, twists, or nonalignments ofgas disks; time-dependent
settling of recently acquired gas; nonaxisymmetric (triaxial)
orbits that may explain why some ellipticals and early-type
spirals have rotational velocities that increase too slowly for
a constant MIL disk; and even holes in the innermost regions
and warps in the outermost regions, so that gas along the
line-of-sight at various nuclear distances and various pro-
jected velocities superposes at a single radial position.
NGC 2683, one of the two spirals with a falling 21-cm
rotation curve discussed above, is a prime example exhibiting
complex inner velocities. I show in Fig. 3 an image of the
Galaxy, the Ha region of the spectrum in the inner 2' with the
underlying stellar continuum removed, the near-nuclear op-
tical velocities, and the 21-cm velocity curve from Casertano
and van Gorkom (20).
Within the inner 60", velocities in NGC 2638 are multival-
ued, extending from the upper maximum velocity down to
velocities negative with respect to the center. The steep
velocity rise implies a massive nucleus, of order5 x 108 Mo.
The upper envelope represents the rotation velocities within
the disk, while at least some of the lower-velocity gas is likely
foreground gas warped out of the plane and intruding into the
line-of-sight. Other low-velocity gas must arise from disk gas
of low angular momentum, probably derived from bulge
stars. None of these details are visible at the lower-resolution
21-cm observations. In fact, these optical observations all lie
closer to the nucleus than the second plotted point on the
21-cm velocity curve; in velocity they rise slightly with
decreased nuclear distance to the velocity V = 220 km/sec at
2" from the nucleus. Thus, a realistic mass deconvolution into
halo, disk, and bulge or central point mass (necessary to
produce the steep nuclear velocity gradient) must be based on
velocities for the inner regions obtained from high-resolution
optical observations and for outer regions on velocities from
high-sensitivity radio observations. Let us now resurrect
Bosma's (5) forgotten suggestion for such composite curves.
Complex inner regions are not rare among galaxies. Rubin
and Kenney (22) have discovered rapidly rotating cores with
kinematic properties distinct from the normal outer rotation
in --25% of the 70 Virgo cluster spirals and ellipticals they
have observed at high-velocity resolution and high spatial
scale. The galaxies are located in all regions of the cluster,
span Hubble types from E through Sc, and have a wide range
of central velocities and viewing angles. For a spiral viewed
close to edge-on, a complex velocity field can be ascribed to
an outer warp projected along the line of sight. For a galaxy
viewed more nearly face-on, the inner region itself must have
a warped or skew or twisted morphology. We are continuing
our analysis of these interesting galaxies. Complex nuclear
regions are especially prevalent in elliptical galaxies, and I
discuss these in Section VI below.
Section III. Dwarf Spirals Contain Mostly Dark Matter
It was recognized a decade ago that low-luminosity galaxies
would be important for learning about dark matter. The
systematics of galaxy rotation curves as a function of lumi-
nosity suggested the query, " . . . for the smaller galaxies,
... Is the luminous matter only a minor component of the
total galaxy mass?" (4). Moreover, the low mass, and con-
sequent low acceleration, suggests that extremely low-mass
galaxies are a fertile testing ground for theories that reject the
existence of dark matter but instead modify Newtonian
gravitational theory.
A major discovery of the last few years is the existence of
low-mass galaxies that are enormously gas rich and in which
the dark mass exceeds the luminous mass by a large amount.
DDO 154, one of the dwarfs richest in neutral hydrogen (23,
24), contains a neutral hydrogen gas disk with radial extent
five times the optical disk. The gas disk is dynamically more
important than the stellar disk; stars constitute only "1% of
the mass. It is the very low velocity gradient beyond the
nucleus, combined with the steep fall in disk luminosity,
which forces the very low-mass disk. Interior to the last
measured velocity, at least 95% of the mass is dark, and MIL
is "50, almost an order of magnitude greater than a typical
type Sc galaxy (within its optical radius).
The existence of dwarf spirals with such a small fraction of
luminous mass is remarkable. One question that naturally
follows, "Is there a galaxy that is totally dark?", has not yet
been answered.
Equally intriguing is the dwarf spiral NGC 1560 (25). Gas
rich, the best-fit model has the stellar and gas masses about
equal, together contributing about one-half the mass of the
dark matter at the last measured point. Especially remarkable
is the close similarity in form with radius ofthe total mass and
the gas mass, particularly as indicated by a dip in both the
velocity and the gas density at -5 kpc. This similarity implies
a close relation between the distribution of the HI gas and the
dark matter. It also makes spiral NGC 1560 especially
interesting for astronomers interested in efforts to modify
Newtonian dynamics to account for flat rotation curves.
Milgrom (26) has proposed modifications to Newtonian dy-
namics (MOND theory), such that at low accelerations the
attractive force varies as llr. In conventional Newtonian
dynamics, "what you see is not what you get"; that is, the
luminous galaxy is embedded in a dark halo whose mass is the
dominant component in producing the observed velocities.
Thus, the velocities predicted by the observed gas density
would be expected to have only an insignificant relation to the
total observed velocities produced by the combined gravita-
tional field of the stars, gas, and (dominant) dark matter.
In contrast, in the MOND theory, "what you see is what
you get," so the agreement of the radial variation of the
observed velocities and the (Newtonian predicted) gas-plus-
stars-predicted velocities is fundamental to the theory. While
few astrophysicists are comfortable with a modified Newto-
nian theory, such theory cannot yet be ruled out on the basis
of the dwarf spirals.
Section IV. Elliptical Galaxies Are Much More Complex
Than We Thought
Velocity Studies from Stellar Absorption Lines. Ten years
ago, discussions ofgalaxy dynamics were based on studies of
spirals only, for little was known of the kinematics of
elliptical galaxies. Ellipticals were unstudied, not because
they were considered simple but because the observations
were hard. Motions in spiral galaxies are relatively easy to
study. Orbital motions are circular, the viewing geometry is
well defined, for a given integration time the prominent gas
optical emission lines are detectable with a signal-to-noise
well above that of the absorption lines, and the neutral
hydrogen gas disks are a prime feature for 21-cm studies.
Thus, it is not surprising that during the 1970s, optical
observations with image tube plus photographic plate and
21-cm observations with new interferometers advanced our
knowledge ofthe kinematics ofdisk galaxies to a degree that
was then not matched for elliptical galaxies. But during the
last decade, knowledge ofdynamics of elliptical galaxies has
started to catch up, due, in part, to efficient digital recording
devices and, in part, to the surprise discovery of significant
emission in some ellipticals, emission that is studied in both
the optical and radio spectral regions. Thus, although an
uncertain viewing angle and complicated triaxial orbits keep
the analysis complex, unexpectedly interesting dynamical
Colloquium Paper: Rubin
5. Proc. Natl. Acad. Sci. USA 90 (1993)
results are now available for a significant number ofelliptical
galaxies.
It is now understood that many bright ellipticals are triaxial
in shape, supported not by their (slow or absent) rotation but
by their anisotropic velocity dispersions (27, 28). But some
ellipticals are rotating, albeit slowly. In a recent study of 22
ellipticals, many of them round, Franx, Illingworth, and
Heckman (29) detect rotation >20 km/sec in 21 of them. Of
these, 60% ofthe ellipticals had their kinematical axis within
100 ofthe photometric minor axis. Yet for six ofthe galaxies,
there is significant minor-axis rotation; in two of these the
minor-axis rotation is more rapid than the major-axis rota-
tion; in two ofthe galaxies the minor-axis rotation is approx-
imately equal to the major-axis rotation. Minor-axis rotation
in triaxial galaxies may be a clue to the formation mechanism
of these galaxies but is currently incompletely understood.
In the inner regions, still more complexity exists. Two of
the ellipticals exhibit skew-rotating small nuclear cores,
another has a counter-rotating core, and a fourth has a core
that rotates perpendicular to the rotation ofthe major axis. In
the Franx et al. (29) study, almost 20% of the galaxies
exhibited kinematically distinct nuclear cores, close to the
fraction Rubin and Kenney observe for spirals and ellipticals
in Virgo.
It is not yet possible to translate absorption line kinemat-
ical studies into statistics ofMIL values for more than a few
ellipticals (30, 31). Within a few scale lengths, MIL values
appear generally constant, and often small, an even tighter
constraint on the amount of dark matter (yet this statement
has the ring of statements made early in the history of
observations of spiral galaxies, when it was expected that
more extended observations would show rotation velocities
to fall). Important observational evidence not discussed here,
such as x-ray observations of halos of ellipticals (32) and
velocities of individual planetary nebula far out in ellipticals
(30, 33), offers impressive evidence that elliptical galaxies do
contain extended dark matter components. While absorption
line studies may not be a major contributor to learning about
the distribution ofextended matter in ellipticals, such studies
will surely teach us important details about the kinematics
and evolutionary history of ellipticals.
Velocity Studies from Gas Emission Lines. Among the many
recent surprises coming from the observations of elliptical
galaxies has been the discovery that a significant number
contain gas disks that can be used to map the gravitational
potential. A collection of rotation curves for elliptical galax-
ies, determined from emission lines, is shown in Fig. 4. Also
shown is the predicted velocity curve for a spherical galaxy
with a normal surface brightness profile; the parallel form for
the predicted and observed relations suggests that MIL is
essentially constant over these radial ranges. A more defin-
itive statement concerning values of MIL as a function of
galaxy radius can be made for only a few special galaxies,
including IC 2006.
IC 2006 (35) is an isolated spherical galaxy surrounded by
a counter-rotating outer ring ofHI. Motions within the galaxy
imply a value ofM/LB = 5 for the core; velocities in the outer
ring suggest that M/LB = 16 overall. Just as for spirals, this
increase in mass per unit of luminosity implies the existence
of nonluminous matter that becomes increasingly dominant
with increased radial distance.
But outer gas disks are rare. More common in elliptical
galaxies are inner disks of ionized gas extending only to a
small fraction of the galaxy radius. Often they are dynam-
ically decoupled from the principal rotation of the galaxy.
Such disks-e.g., NGC 5128 (36) and NGC 1316 (37)-are
now understood as the product of the acquisition of a small
gas-rich system. This gas and dust will require numerous
orbital periods to reach dynamical equilibrium and mean-
R/Re
FIG. 4. Rotation curves for six elliptical galaxies plotted as a
function of Re, the half light radius (R = radius) (34). Velocities
(VROT) are measured from their gas disks. The dotted curve is the
rotation curve for a spherical galaxy with a normal r1/4 surface
brightness profile.
while will be observed as a warped and twisted disk with
complex internal kinematics.
NGC 5077 is an elliptical galaxy with a disk of ionized gas
along its minor axis, a not uncommon form for ellipticals. A
detailed study of its gas velocities (34) shows that either
triaxial or oblate mass models with constant MIL can match
the velocity fields of the stars, the gas, and the central
velocity dispersion. Even though the rotation velocities in-
crease slowly from the nucleus, such velocities can arise from
gas in elliptical orbits; it is not necessary to invoke an MIL
increasing with radial distance. Moreover, models with the
gas located in a time-evolving polar ring predict that the ring
becomes enormously twisted, so that lines of sight intersect
the surface two or three times, thus giving rise to the complex
multivalued velocities they observe in the gas.
N-body simulations, too, produce small counter-rotating
nuclear disks. Barnes and Hernquist (38, 39) show that a
prograde merger of two gas-rich stellar disks produces a
galaxy photometrically indistinguishable from an elliptical,
which contains a kinematically distinct but small (=800 pc)
central gas core. Such cores may be skew or counter rotating,
for the tidal trauma has destroyed all knowledge of the
previous orientation ofthe gas. However, the more extended
gas, settled in a disk, rotates prograde.
Many comprehensive reviews of the current state of ob-
servations and understanding of the dynamics of elliptical
galaxies are available (40, 41). It is clear that the freedom
afforded by triaxial orbits combined with viewing angles
offers wide possibilities for models. Adding warped, twisted,
precessing, and time-dependent gas disks increases the range
ofparameters. Arriving at the correct model may be difficult,
for increasingly detailed observations may only add to the
complexity, rather than constrain the range of parameters.
However, the galaxy-by-galaxy studies that are continuing at
an accelerating pace will ultimately make clear the evolu-
tionary importance for ellipticals ofrelatively newly acquired
material; with luck we will also learn about the overall
distribution of matter in elliptical galaxies.
NGC 4550. Recently, I have been studying, in collaboration
with Graham and Kenney (42), the kinematics of spiral and
elliptical galaxies in the Virgo cluster. NGC 4550 is a fea-
tureless E7/SO galaxy located in the core ofthe Virgo cluster
only 1.2° from M87 (Fig. 5). We have discovered a unique
pattern ofstellarand gas kinematics over the inner 30% ofthe
optical galaxy. The stellar disk contains two interpenetrating
components; one system is rotating prograde and one system,
cospatial with the first, is rotating retrograde. A rotating gas
disk is coincident with one ofthe stellar disks. Thus, instead
4818 Colloquium Paper: Rubin
6. Proc. Natl. Acad. Sci. USA 90 (1993) 4819
A
B
o 800U
.4 700
._
.o 600
10 500
, 400
3.0
300,
- NGC 4550
_ *~~~200
_ * 4-r
- 0 200
_ ~ ~~~~>4-rr
S
I s
W
l I
-40 -20
Distance fron
C
oblate spheroids. Thus, it is axisymmetric, flattened by
rotation, with no apparent morphological or other kinemat-
ical peculiarities. Our limited material does not permit a.tl ...i ..:..
detailed model, although absorption line intensities and mea-
P fA v
~~~~sured velocities suggest that the two stellar systems have
jbr^X. i approximately similar dynamical properties, and are approx-
imately coplanar, and absorption line strengths within the
inner few kpc are only slightly stronger in the prograde disk.
Virtually all of the possibilities for the origin of the kine-
.........
.matics in NGC 4550 require the acquisition of secondary
material afterthe initial stellar disk ofNGC 4550 was in place.
We propose the folowing likely scenario. A retrograde gas
mass, probably small compared with the mass ofNGC 4550,
is captured at some angle intermediate between polar and
disk, settles to the disk via dissipation caused partly by
differential precession (44,45), and forms most stars after the.t '. *: ..~~~~~~~ ~~~~~~~~~~~........ ...::
gas has settled to the plane. Such an uncommon system can
be understood as an "equatorial ring" (46)-i.e., a disk, an
alternative end point to the polar ring sequence.
The lack of morphological distortions and the presence of
strong metal absorption lines suggest that the time elapsed
tPA-358° - since the acquisition is at least 109 yr, allowing time for the
_PA=35 gas to come to equilibrium in the combined gravitational
)-inch Ha,[N],[S] emissio potential and for the merger population to form and age. An
neineh Ha.[N] emrissio°n ~ age of 109 yr corresponds to :10 orbital periods at the
-inheHa absorption - half-light radius (16") and to 6 orbital periods at the limits ofneter Ha absorption
our velocity measurements, sufficient to destroy the mor-
phological evidence of the event.
Cospatial stellar systems rotatingboth clockwise and coun-
ft *z, _ terclockwise are more stable than unidirectional disks (S.
Tremaine, private communication) and have been discussed
4 by Araki (47). Earlier, Lynden-Bell (48) imagined a Maxwell
demon who reversed the direction (and violated conservation
t +N
of angular momentum) of one-half the stars in a spherical
_~N cluster to form two interpenetrating stable counter-rotating
| | I- spherical clusters. In Toomre's (49) discussion of axially
0 20 40 symmetric flattened isothermal galaxy models, he investi-
n nucleus, arcsec gated sets of models in which one-half of the stars rotate
prograde, one-half rotate retrograde, noting that nothing in
the analysis restricts the disk stars to orbit only prograde.
The discovery of galaxies like NGC 4550 confirms yet
again the enormous variety within elliptical galaxies;
Toomre's (49) models are no longer just the "elegant curi-
osities" he imagined. I envy observers living within NGC
4550 for the interesting stellar dynamics they are encounter-
ing.
FIG. 5. (Top) Virgo galaxies NGC 4550 and NGC 4551 from an
r-band image taken by C. Bailyn and Y.-C. Kim at the Cerro Tololo
InterAmerican Observatory 0.9-m telescope. There is no evidence of
morphological peculiarity in either galaxy. (Middle) Major-axis ve-
locities in NGC 4550, from the stellar Ha absorption (open symbols)
and from the counter-rotating emission disk (filled symbols). The
counter-rotating stellar disk absorption at Ha is masked by the
emission, except for that barely showing at -15' N. (Bottom)
Absorption lines from prograde and retrograde stellar disks (here
shown dark) in the region blueward of[OIII]5007 (bright line at right).
Note the X-shaped appearance of the absorption lines as they cross
the nucleus.
of a normal spectrum composed of many single absorption
lines, we observe each absorption line to be double, with the
two members opposite in velocity and crossing at the nu-
cleus. The two cospatial counter-rotating disks extend over
a sizable portion of the galaxy.
A value of V/lo = 1.4, coupled with an ellipticity, 1 - b/a
= 0.7, places NGC 4550 in the Binney (43) region of rotating
Section V. M/L 100 for the Perseus Cluster,
a "Low" Value
Nearly 60 years ago, Zwicky (50) and Smith (51) discovered
that individual galaxies in the Coma and Virgo clusters of
galaxies have velocities so large with respect to the mean
cluster velocity that their mutual gravitational attraction
would not be sufficient to bind the clusters. Hence, either
clusters are expanding and galaxies are dispersing (contrary
to observation) or the total mass in the clusters is larger than
that implied by the luminous material. Zwicky called this the
"missing mass." For many years, this conclusion was rele-
gated to the "things we don't understand" file and assumed
to be a mysterious property of galaxy clusters. Not until we
discovered the need for dark matter in individual galaxies did
these early observations join the mainstream ofastronomical
research.
Now that we recognize that dark matter is the major
constituent ofclusters, we must quantify the amount ofdark
matter if we are to set observational limits on the mass and
the ultimate fate of the universe. Is MIL averaged over a
cluster as large as 500 or 1000, implying that MIL rises with
the size ofthe region included, or is it closer to the value 100
Colloquium Paper: Rubin
7. Proc. Natl. Acad. Sci. USA 90 (1993)
derived for binaries and groups? During the last decade or
two, attempts to answer this question were made generally
via a virial analysis, an analysis that assumes galaxy mem-
bership in the cluster is known, that the distribution of mass
follows the distribution ofgalaxies, that the cluster is relaxed,
that the galaxy velocity distribution is isotropic, and that the
second time derivative ofthe moment ofinertia is zero (which
would not be the case, for example, if galaxy halos were
centered elsewhere than at the galaxy centers). Some years
ago, from optical data for the Perseus cluster, Kent and
Sargent (52) derived the large value of MIL 300.
In an important recent work, Eyles et al. (53) analyze x-ray
images of the Perseus cluster obtained with spatial resolu-
tions of 12' and 3' in the energy range 2.5-32 keV (1 eV =
1.602 x 10-19 J) during the Spacelab 2 mission. Because the
x-ray-emitting gas in a cluster is expected to be in hydrostatic
equilibrium, kT plays the role ofmV2 in the virial analysis and
is a good tracer of the cluster gravitational potential. From
the x-ray data, Eyles et al. (53) find that the total gravitational
cluster mass is 30-65% less than that obtained from the
application of the virial theorem, in part because the distri-
bution of mass does not follow the distribution of galaxies.
Moreover, while the dark matter is more centrally condensed
than the galaxies and is the dominant mass component out to
=1 Mpc, the x-ray gas is less so. The fraction ofthe total mass
that is in the form of the x-ray-emitting gas increases with
radius, dominating beyond 1 Mpc and reaching >55% of the
total mass and still rising at the limits of the observations.
As a result of this analysis, M/Lv is of order 100 at large
radii (rather than 300), with values between 55 and 145
permitted by the range of models and of statistical and
systematic errors. Although based on only a single cluster,
this result strengthens earlier suggestions (54) that there is not
strong evidence for very large values of MIL for systems of
very large size. New x-ray observations from ROSAT sat-
ellite are expected to enlarge the sample ofgalaxy clusters for
which determinations of the mass distribution and, conse-
quently, MIL are available.
Conclusions: MIL 100 Wi Saisfy,Al
Dynamical Observations
To discuss the observed dynamical values of MIL and the
mass density ofthe universe, I start with a few definitions. I
adopt Ho = 50 km sec-l-Mpc-1. Then:
Critical mass density Pc = 3H2/8rG 5 x 10-30 g.cm3
7 x 1010 MG-Mpc-3,
Present mass density p = 3HW2/8irG, where Q = P/Pc,
Luminosity density LB = 108 LO-Mpc3.
Hence, MILB = p/LB 750 Q, where the blue luminosity
density, surely an uncertain number, is takenfrom Efstathiou
et al. (55). Thus, a value of M/LB near 750 will close the
universe-i.e., Q will be unity, and the gravitational attrac-
tion of the mass within the universe will just halt the expan-
sion. In the values that follow, I have transformed published
values to the adopted Ho and to the blue spectral band.
It had earlier been suspected that observed values ofMIL
increase with increased enclosed volume. However, current
available data suggest that this is not so, and M/L 100 is
a limiting value required by dynamical systems on scales
from 0.1 to 200 Mpc (except the highly uncertain values
derived in studies of large-scale motions-e.g., ref. 56). This
situation is illustrated in Fig. 6, a plot of the dynamically
determined M/L values versus the appropriate radial scale
for our galaxy, galaxies, binary galaxies, the Local Group,
clusters of galaxies, and for gravitationally induced infall
motions. For scales larger than galaxies-i.e., >100 kpc, all
systems have a constant ratio ofMIL. On the smaller scales
(galaxies), the increasing dominance of the dark matter
produces values of MIL that increase with radial scale. All
Radial scale, kpc
FIG. 6. Mass-to-blue light ratio as a function of the size of the system for galaxies, binaries, clusters, and larger systems, based loosely on
and extended from Oemler (57). References to individual values come from Rubin (3, 4); Forman et al. (32); Kulessa and Lynden-Bell (9);
Schweizer (58); Huchra and Geller (59); Cowie et al. (60); Eyles et al. (53); Peebles (13); van der Kruit (11); Davis and Peebles (61); Aaronson
et al. (62); and Schramm (63); values are all transformed to H. = 50 km sec-l-Mpc-1 and blue magnitudes. A value of MIL = 750 is equivalent
to Q = 1. [For H. = 100 km sec- Mpc-1 values of MIL increase by a factor of 2, and MIL (closure) = 1500. The form of the plot relative to
[l remains unchanged]. Error bars are generic estimates. Note that beyond =100 Mpc, values of M/L do not continue to rise but are constant
near 100, independent ofthe size ofthe dynamical system. Dwarfspirals and irregulars, not plotted here, have MIL values ranging from =20-50
and radii near 10 kpc. However, their poorly known distances introduce large uncertainties in both quantities.
4820 Colloquium Paper: Rubin
8. Proc. Natl. Acad. Sci. USA 90 (1993) 4821
values fit comfortably within a universe in which the lumi-
nous mass in galaxies comprises only 1% of the closure
density; the mass required by the dynamics is ten times this
or 10% of the closure density, but not higher. Moreover,
these values do not exceed the nucleosynthetic limit on the
mass density of baryons in the universe (e.g., ref. 63).
Thus, observations tell us that we may inhabit a low-
density universe in which the gravitational attraction of the
enclosed baryonic matter is not sufficient to halt the expan-
sion. However, theoretical cosmologists like Guth and
Schramm and Turner tell us that models require that Q is
unity, that the additional 90% of the universe is still unde-
tected, and that it is required to consist ofdark, nonbaryonic
matter. To reconcile the views of the observers with the
dreams of the theorists will require heroic observational,
laboratory, and theoretical efforts but will constitute a fun-
damental step in teaching us more about the complicated
universe in which we find ourselves.
I thank my colleagues John Graham, Jeff Kenney, and Francois
Schweizer for valuable help and support; Kitt Peak and Palomar
Observatories for telescope time; and Drs. F. Bertola, F. Briggs, S.
Casertano, and J. Kormendy for permission to reproduce their
figures.
1. Ostriker, J. P., Peebles, P. J. E. & Yahill, A. (1974) Astrophys.
J. Lett. 193, Li-L4.
2. Ostriker, J. P. & Peebles, P. J. E. (1973) Astrophys. J. 186,
467-480.
3. Rubin, V. C., Ford, W. K. & Thonnard, N. (1978) Astrophys.
J. Lett. 225, L107-L111.
4. Rubin, V. C., Ford, W. K. & Thonnard, N. (1980) Astrophys.
J. 238, 471-487.
5. Bosma, A. (1978) Dissertation (Univ. ofGroningen, The Neth-
erlands).
6. Faber, S. M. & Gallagher, J. S. (1979) Annu. Rev. Astron.
Astrophys. 17, 135-187.
7. Kormendy, J. & Knapp, G. R., eds. (1987) in Dark Matter in
the Universe (Reidel, Dordrecht, The Netherlands).
8. Trimble, V. (1987) Annu. Rev. Astron. Astrophys. 25, 425-472.
9. Kulessa, A. S. & Lynden-Bell, D. (1992) Mon. Not. R. Astron.
Soc. 255, 105-118.
10. Schmidt, M. (1985) The Milky Way Galaxy, Int. Astron. Union
Symposium 106 (Reidel, Dordrecht, The Netherlands), pp.
75-84.
11. van der Kruit, P. C. (1990) The Milky Way as a Galaxy,
Saas-Fee Advanced Course 19 (Univ. Science Books, CA), pp.
185-226.
12. Zaritsky, D., Smith, R., Frenk, C. & White, S. D. M. (1993)
Astrophys. J., in press.
13. Peebles, P. J. E. (1990) Astrophys. J. 362, 1-13.
14. Kamphuis, J. & Briggs, F. (1992) Astron. Astrophys. 253, 335.
15. Kormendy, J. (1988) in Origin, Structure, and Evolution of
Galaxies, ed. Zhi, F. L. (World Sci., Singapore), pp. 252-280.
16. van Albada, T. S., Bahcall, J. B., Begeman, K. & Sancisi, R.
(1985) Astrophys. J. 295, 305-313.
17. Kent, S. M. (1988) Astron. J. 96, 514-527.
18. Athanassoula, E., Bosma, A. & Papaioannou, S. (1987) Astron.
Astrophys. 179, 23-40.
19. Kalnajs, A. J. (1985) in The Internal Kinematics andDynamics
of Galaxies, ed. Athanassoula, E. (Reidel, Dordrecht, The
Netherlands), pp. 87-88.
20. Casertano, S. & van Gorkom, J. H. (1991) Astron. J. 101,
1231-1241.
21. Whitmore, B. C., Forbes, D. A. & Rubin, V. C. (1988) Astro-
phys. J. 333, 542-560.
22. Rubin, V. C. & Kenney, J. D. P. (1991) Bull. Am. Astron. Soc.
23, 1339 (abstr.).
23. Carignan, C. & Freeman, K. C. (1988) Astrophys. J. Lett. 332,
L33-L36.
24. Carignan, C. & Beaulieu, S. (1989) Astrophys. J. 347, 760-770.
25. Broeils, A. H. (1992) Astron. Astrophys. 256, 19-32.
26. Milgrom, M. (1983) Astrophys. J. 270, 365-389.
27. Illingworth, G. (1977) Astrophys. J. Lett. 218, L43-L47.
28. Davies, R. L., Efstathiou, G., Fall, S. M., Illingworth, G. &
Schechter, P. L. (1983) Astrophys. J. 266, 41-57.
29. Franx, M., Illingworth, G. & Heckman, T. (1990) Astrophys. J.
344, 613-636.
30. Kent, S. M. (1990) in Evolution ofthe Universe ofGalaxies, ed.
Kron, R. G. (Brigham Young Univ., Provo, UT), pp. 109-118.
31. de Zeeuw, T. (1992) in Morphological and Physical Classifi-
cation ofGalaxies, eds. Longo, G., Capaccioli, M. & Busarello,
G. (Kluwer, Dordrecht, The Netherlands), pp. 139-151.
32. Forman, W., Jones, C. & Tucker, W. (1985) Astrophys. J. 293,
102-119.
33. Ciardullo, R., Jacoby, G. H. & Ford, H. C. (1989) Astrophys.
J. 344, 715-725.
34. Bertola, F., Bettoni, D., Danzinger, J., Sadler, E., Sparke, L.
& de Zeeuw, T. (1991) Astrophys. J. 373, 369-390.
35. Schweizer, F., van Gorkom, G. H. & Seitzer, P. (1989) Astro-
phys. J. 338, 770-788.
36. Graham, J. A. (1979) Astrophys. J. 232, 60-73.
37. Schweizer, F. (1980) Astrophys. J. 237, 308-318.
38. Barnes, J. E. & Hernquist, L. E. (1991) Astrophys. J. 370,
L65-L68.
39. Hernquist, L. E. & Barnes, J. E. (1991) Nature (London) 354,
210-212.
40. Wielen, E. R., ed. (1990) in Dynamics and Interactions of
Galaxies (Springer, Heidelberg).
41. de Zeeuw, T. & Franx, M. (1991) Annu. Rev. Astron. Astro-
phys. 29, 239-274.
42. Rubin, V. C., Graham, J. A. & Kenney, J. P. D. (1992) Astro-
phys. J. 394, L9-L12.
43. Binney, J. (1978) Mon. Not. R. Astron. Soc. 183, 501-514.
44. Sparke, L. S. (1986) Mon. Not. R. Astron. Soc. 219, 657-670.
45. de Zeeuw, T. (1990) in Dynamics andInteractions ofGalaxies,
ed. Wielen, E. R. (Springer, Heidelberg), pp. 263-266.
46. Schechter, P. L. (1990) in Dynamics and Interactions ofGal-
axies, ed. Wielen, E. R. (Springer, Heidelberg), pp. 508-513.
47. Araki, S. (1987) Astron. J. 94, 99-105.
48. Lynden-Bell, D. (1960) Mon. Not. R. Astron. Soc. 120, 204-
213.
49. Toomre, A. (1982) Astrophys. J. 259, 535-543.
50. Zwicky, F. (1933) Helv. Phys. Acta 6, 110.
51. Smith, S. (1936) Astrophys. J. 83, 23.
52. Kent, S. M. & Sargent, W. L. W. (1983)Astron. J. 88,697-708.
53. Eyles, C. J., Watt, M. P., Bertram, D., Church, M. J., Pon-
man, T. J., Skinner, G. K. & Willmore, A. P. (1991)Astrophys.
J. 376, 23-32.
54. Davis, M. (1987) in Dark Matter in the Universe, eds. Kor-
mendy, J. & Knapp, G. R. (Reidel, Dordrecht, The Nether-
lands), pp. 97-109.
55. Efstathiou, G., Ellis, R. S. & Peterson, B. A. (1988) Mon. Not.
R. Astron. Soc. 232, 431-461.
56. Guth, A. H. (1993) Proc. Natl. Acad. Sci. USA 90, 4871-4877.
57. Oemler, A. (1988) in The Minnesota Lectures on Clusters of
Galaxies and Large-Scale Structure, ed. Dickey, J. M.
(Brigham Young Univ., Provo, UT), pp. 19-38.
58. Schweizer, L. (1988) Astrophys. J. Suppl. Ser. 64, 427-457.
59. Huchra, J. & Geller, M. (1982) Astrophys. J. 257, 423-437.
60. Cowie, L. L., Henriksen, M. & Mushotzky, R. (1987) Astro-
phys. J. 317, 593-600.
61. Davis, M. & Peebles, P. J. E. (1983) Astrophys. J. 267, 465-
482.
62. Aaronson, M., Huchra, J., Mould, J., Schechter, P. L. & Tully,
R. B. (1982) Astrophys. J. 258, 64-76.
63. Schramm, D. N. (1993) Proc. Natl. Acad. Sci. USA 90, 4782-
4788.
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