143 research outputs found
CO distribution in southern S0 galaxies
With the Swedish ESO Submillimeter Telescope (SEST), researchers observed 7 S0 galaxies at 2.6 mm, and detected CO emission in five. Observing four offset positions per galaxy at greater than or equal to 40 seconds from the center, they find significantly extended CO emission in almost all cases. The (lower limits to) H2 masses of several times 10(exp 8) solar magnitude amount to 0.2-0.3 times the HI mass in 4 or 5 galaxies
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The distribution of the interstellar medium in the Scd galaxy NGC 6946 and in a sample of dwarf irregular galaxies.
We have compared the atomic and molecular hydrogen distributions in the spiral galaxy NGC 6946 and in 15 dwarf irregular galaxies. We have made observations of the CO J = 1 0 transition to obtain a fully sampled map to a radium of 10 kpc in NGC 6946, using the 14-meter telescope of the Five College Radio Astronomy Observatory, and have also completely mapped the 21-cm line of neutral hydrogen with the Very Large Array. We have also made CO observations at FCRAO in 15 dwarf irregular galaxies spanning a wide range of infrared luminosity and color. In NGC 6946 the CO and HI maps were used to investigate the radial and azimuthal variations in the atomic and molecular gas. The H\sb2 mass is found to dominate the ISM in the inner disk of this galaxy. The H\sb2, H, radio continuum, and blue light radial profiles are found to be centrally peaked with exponential falloffs, while the HI distribution is much flatter, with a central depression. We infer a constant efficiency of star formation with radius in NGC 6946 and suggest that HI plays only a passive role in the star formation process of this galaxy. Spiral arm enhancements in the H\sb2 surface densities, H surface brightnesses, and (H\sb2)/(H) ratios are observed. The presence of these arm enhancements, as well as enhancements in the gas surface densities in interarm regions is consistent with a picture in which cloud-cloud collisions play an important role in the star formation process in NGC 6946. An observed strong increase in the (H)/(H\sb2) ratio with increasing arm amplitude indicates that the massive star formation efficiency is a sensitive function of spiral arm strength. For the dwarf galaxies, high observed L\sb{\rm IR}/L\sb{\rm CO} ratios in the blue systems are indicative of a present burst of star formation, while the red systems exhibit low L\sb{\rm IR}/L\sb{\rm CO} ratios. The M(H\sb2)/M(HI) ratios are smaller in the blue dwarfs than in either the red systems or in NGC 6946. We suggest that a low metallicity, low volume density, and an increased UV radiation field are responsible for the low H\sb2/HI ratios in active dwarf galaxies
The azimuthal and radial distributions of HI and H2 in NGC 6946
A study was completed of the atomic and molecular components of the ISM in NGC 6946. The distribution of molecular clouds was determined from a fully sampled CO map of the inner disk using the 14-meter telescope of the FCRAO. The distribution of atomic gas was derived from VLA observations at 40" resolution in the D configuration. When comparing the global CO and HI properties with other components of the galaxy, it was found that the azimuthally averaged radial distributions of CO, H-alpha, radio continuum and blue light all exhibit similar roughly exponential falloffs, while the azimuthally averaged HI surface densities vary by only a factor of 2 out to R = 16 kpc. This indicates that while the H-alpha/CO ratio is approximately constant with radius, the CO/HI ratio decreases by a factor of 30 from the center of the galaxy to R = 10 kpc
High resolution near-infrared imaging of submillimeter galaxies
We present F110W (~J) and F160W (~H) observations of ten submillimeter
galaxies (SMGs) obtained with the Hubble Space Telescope's (HST's) NICMOS
camera. Our targets have optical redshifts in the range 2.20<z<2.81 confirmed
by millimeter CO or mid-IR spectroscopy, guaranteeing that the two bands sample
the rest-frame optical with the Balmer break falling between them. Eight of ten
are detected in both bands, while two are detected in F160W only. We study
their F160W morphologies, applying a maximum-deblending detection algorithm to
distinguish multiple- from single-component configurations, leading to
reassessments for several objects. Based on our NICMOS imaging and/or previous
dynamical evidence we identify five SMGs as multiple sources, which we
interpret as merging systems. Additionally, we calculate morphological
parameters asymmetry (A) and Gini coefficient (G); thanks to our sample's
limited redshift range we recover the trend that multiple-component,
merger-like morphologies are reflected in higher asymmetries. We analyze the
stellar populations of nine objects with F110W/F160W photometry, using archival
HST optical data when available. For multiple systems, we are able to model the
individual components that build up an SMG. With the available data we cannot
discriminate among star formation histories, but we constrain stellar masses
and mass ratios for merger-like SMG systems, obtaining a mean
log(M_*/M_sun)=10.9+/-0.2 for our full sample, with individual values
log(M_*/M_sun)~9.6-11.8. The morphologies and mass ratios of the least and most
massive systems match the predictions of the major-merger and cold accretion
SMG formation scenarios, respectively, suggesting that both channels may have a
role in the population's origin.Comment: 41 pages preprint, 3 figures, published in ApJ on 2013 May 1
The Evolution of the Star-forming Interstellar Medium across Cosmic Time
Over the past decade increasingly robust estimates of the dense molecular gas
content in galaxy populations between redshift 0 and the peak of cosmic
galaxy/star formation from redshift 1-3 have become available. This rapid
progress has been possible due to the advent of powerful ground-based, and
space telescopes for combined study of several millimeter to far-IR, line or
continuum tracers of the molecular gas and dust components. The main
conclusions of this review are:
1. Star forming galaxies contained much more molecular gas at earlier cosmic
epochs than at the present time.
2. The galaxy integrated depletion time scale for converting the gas into
stars depends primarily on z or Hubble time, and at a given z, on the vertical
location of a galaxy along the star-formation rate versus stellar mass
"main-sequence" (MS) correlation.
3. Global rates of galaxy gas accretion primarily control the evolution of
the cold molecular gas content and star formation rates of the dominant MS
galaxy population, which in turn vary with the cosmological expansion. A second
key driver may be global disk fragmentation in high-z, gas rich galaxies, which
ties local free-fall time scales to galactic orbital times, and leads to rapid
radial matter transport and bulge growth. Third, the low star formation
efficiency inside molecular clouds is plausibly set by super-sonic streaming
motions, and internal turbulence, which in turn may be driven by conversion of
gravitational energy at high-z, and/or by local feedback from massive stars at
low-z.
4. A simple 'gas regulator' model is remarkably successful in predicting the
combined evolution of molecular gas fractions, star formation rates, galactic
winds, and gas phase metallicities.Comment: To be published in Annual Reviews of Astronomy and Astrophysic
A Compact Starburst Core in the Dusty Lyman Break Galaxy Westphal-MD11
Using the IRAM Plateau de Bure Interferometer, we have searched for CO(3-2)
emission from the dusty Lyman break galaxy Westphal-MD11 at z = 2.98. Our
sensitive upper limit is surprisingly low relative to the system's 850 um flux
density and implies a far-IR/CO luminosity ratio as elevated as those seen in
local ultraluminous mergers. We conclude that the observed dust emission must
originate in a compact structure radiating near its blackbody limit and that a
relatively modest molecular gas reservoir must be fuelling an intense nuclear
starburst (and/or deeply buried active nucleus) that may have been triggered by
a major merger. In this regard, Westphal-MD11 contrasts strikingly with the
lensed Lyman break galaxy MS1512-cB58, which is being observed apparently
midway through an extended episode of more quiescent disk star formation.Comment: 5 pages, 1 figure (emulateapj), accepted by ApJ
Molecular Gas and Star Formation in the Host Galaxy of I Zw 1
A recent analysis of high angular resolution NIR imaging and spectroscopic
data in conjunction with Plateau de Bure interferometric mm-line observations
indicate the presence of a circum-nuclear starburst ring of about 1.5 arcsec
(1.5 kpc) diameter in I Zw 1. High angular resolution NIR imaging, using the
MPE SHARP camera at the ESO NTT, HST V-band images, as well as NIR spectroscopy
with MPE 3D provide an improved analysis of the star formation activity in the
disk and nucleus of I Zw 1. We present first results from subarcsecond
interferometric imaging in the 12CO(2-1) line using the Plateau de Bure
Interferometer.Comment: Contributed talk presented at the Joint MPE,AIP,ESO workshop on
NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also
available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho
COLD GASS, an IRAM Legacy Survey of Molecular Gas in Massive Galaxies: III. Comparison with semi-analytic models of galaxy formation
We compare the semi-analytic models of galaxy formation of Fu et al. (2010),
which track the evolution of the radial profiles of atomic and molecular gas in
galaxies, with gas fraction scaling relations derived from the COLD GASS survey
(Saintonge et al 2011). The models provide a good description of how condensed
baryons in galaxies with gas are partitioned into stars, atomic and molecular
gas as a function of galaxy stellar mass and surface density. The models do not
reproduce the tight observed relation between stellar surface density and
bulge-to-disk ratio for this population. We then turn to an analysis of
the"quenched" population of galaxies without detectable cold gas. The current
implementation of radio-mode feedback in the models disagrees strongly with the
data. In the models, gas cooling shuts down in nearly all galaxies in dark
matter halos above a mass of 10**12 M_sun. As a result, stellar mass is the
observable that best predicts whether a galaxy has little or no neutral gas. In
contrast, our data show that quenching is largely independent of stellar mass.
Instead, there are clear thresholds in bulge-to-disk ratio and in stellar
surface density that demarcate the location of quenched galaxies. We propose
that processes associated with bulge formation play a key role in depleting the
neutral gas in galaxies and that further gas accretion is suppressed following
the formation of the bulge, even in dark matter halos of low mass.Comment: 12 figures, accepted for publication in MNRAS, the COLD GASS data is
available at http://www.mpa-garching.mpg.de/COLD_GASS/data.shtm
Star-Forming Galaxies at z~2: An Emerging Picture of Galaxy Dynamics and Assembly
In these proceedings, we summarize recent results from our "SINS" VLT/SINFONI
integral-field survey, focusing on the 52 detected UV/optically-selected
star-forming galaxies at z~2. Our H-alpha emission-line imaging and kinematic
data of these systems illustrates that a substantial fraction (> 1/3) of these
galaxies are large, rotating disks and that these disks are clumpy, thick, and
forming stars rapidly. We compare these systems to local disk scaling relations
and find that the backbones of these relations are already in place at z~2.
Detailed analysis of the large disks in our sample provides strong evidence
that this population cannot result from a merger-dominated formation history
and instead must be assembled by the smooth but rapid inflow of gas along
filaments. These systems will then secularly evolve from clump-dominated disks
to bulge-dominated disks on short timescales, a phenomenon that is observed in
our SINS observations and is consistent with predictions from numerical
simulations. These results provide new and exciting insights into the formation
of bulge-dominated galaxies in the local Universe.Comment: 7 pages, to appear in the proceedings of "Galaxy Evolution: Emerging
Insights and Future Challenges," Austin, TX, 11-14 Nov 200
Physical Properties of Molecular Clouds at 2 parsec Resolution in the Low-Metallicity Dwarf Galaxy NGC 6822 and the Milky Way
We present the ALMA survey of CO(2-1) emission from the 1/5 solar
metallicity, Local Group dwarf galaxy NGC 6822. We achieve high (0.9 arcsec ~ 2
pc) spatial resolution while covering large area: four 250 pc x 250 pc regions
that encompass ~2/3 of NGC 6822's star formation. In these regions, we resolve
~150 compact CO clumps that have small radii (~2-3 pc), narrow line width (~1
km/s), and low filling factor across the galaxy. This is consistent with other
recent studies of low metallicity galaxies, but here shown with a 15 times
larger sample. At parsec scales, CO emission correlates with 8 micron emission
better than with 24 micron emission and anti-correlates with Halpha, so that
PAH emission may be an effective tracer of molecular gas at low metallicity.
The properties of the CO clumps resemble those of similar-size structures in
Galactic clouds except of slightly lower surface brightness and CO-to-H2 ratio
~1-2 times the Galactic value. The clumps exist inside larger atomic-molecular
complexes with masses typical for giant molecular cloud. Using dust to trace H2
for the entire complex, we find CO-to-H2 to be ~20-25 times the Galactic value,
but with strong dependence on spatial scale and variations between complexes
that may track their evolutionary state. The H2-to-HI ratio is low globally and
only mildly above unity within the complexes. The SFR-to-H2 ratio is ~3-5 times
higher in the complexes than in massive disk galaxies, but after accounting for
the bias from targeting star-forming regions, we conclude that the global
molecular gas depletion time may be as long as in massive disk galaxies.Comment: Accepted for publication in The Astrophysical Journal; 22 pages, 10
figures, 7 table
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