90 research outputs found
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
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
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
Studying the Dynamics of Star Forming and IR Luminous Galaxies with Infrared Spectroscopy
With the advent of efficient near-IR spectrometers on 10m-class telescopes,
exploiting the new generation of low readout noise, large format detectors, OH
avoidance and sub-arcsecond seeing, 1-2.4 micron spectroscopy is becoming a key
means of obtaining detailed galaxy dynamics and for studies of high-z galaxies.
In the following we present the results of three recent IR spectroscopy studies
on the dynamics of ULIRG mergers, super star clusters in the Antennae, and on
the properties of the rotation curves of z~1 disk galaxies, carried out with
ISAAC on the VLT and NIRSPEC on the Keck.Comment: To appear in the Proceedings of the ESO Workshop "The Mass of
Galaxies at Low and High Redshift", R. Bender and A. Renzini Eds.
(Springer-Verlag
Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z~2 III: Far-IR to Radio Properties and Optical Spectral Diagnostics
We present the far-IR, millimeter, and radio photometry as well as optical
and near-IR spectroscopy of a sample of 48 z~1-3 Spitzer-selected ULIRGs with
IRS mid-IR spectra. Our goals are to compute their bolometric emission, and to
determine both the presence and relative strength of their AGN and starburst
components. We find that strong-PAH sources tend to have higher 160um and 1.2mm
fluxes than weak-PAH sources. The depth of the 9.7um silicate feature does not
affect MAMBO detectability. We fit the far-IR SEDs of our sample and find an
average ~7x10^{12}Lsun for our z>1.5 sources. Spectral decomposition
suggests that strong-PAH sources typically have ~20-30% AGN fractions. Weak-PAH
sources by contrast tend to have >~70% AGN fractions, with a few sources having
comparable contributions of AGN and starbursts. The optical line diagnostics
support the presence of AGN in the bulk of the weak-PAH sources. With one
exception, our sources are narrow-line sources, show no obvious correspondence
between the optical extinction and the silicate feature depth, and, in two
cases, show evidence for outflows. Radio AGN are present in both strong-PAH and
weak-PAH sources. This is supported by our sample's far-IR-to-radio ratios (q)
being consistently below the average value of 2.34 for local star-forming
galaxies. We use survival analysis to include the lower-limits given by the
radio-undetected sources, arriving at =2.07+/-0.01 for our z>1.5 sample. In
total, radio and, where available, optical line diagnostics support the
presence of AGN in 57% of the z>1.5 sources, independent of IR-based
diagnostics. For higher-z sources, the AGN luminosities alone are estimated to
be >10^{12}Lsun, which, supported by the [OIII] luminosities, implies that the
bulk of our sources host obscured quasars.Comment: 22 pages, 14 figures, accepted for publication in Ap
On the Relation Between Black Hole Mass and Velocity Dispersion in Type 1 and Type 2 AGN
We present results from infrared spectroscopic projects that aim to test the relation
between the mass of a black hole M_(BH) and the velocity dispersion of the stars in its host-galaxy
bulge. We demonstrate that near-infrared, high-resolution spectroscopy assisted by adaptive
optics is key in populating the high-luminosity end of the relation. We show that the velocity
dispersions of mid-infrared, high-ionization lines originating from gas in the narrow-line region
of the active galactic nucleus follow the same relation. This result provides a way of inferring
MBH estimates for the cosmologically significant population of obscured, type 2 AGN that can
be applicable to data from spectrographs on next-generation infrared telescopes
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