235 research outputs found
Evolution of the atomic and molecular gas content of galaxies in dark matter haloes
We present a semi-empirical model to infer the atomic and molecular hydrogen
content of galaxies as a function of halo mass and time. Our model combines the
SFR-halo mass-redshift relation (constrained by galaxy abundances) with
inverted SFR-surface density relations to infer galaxy H I and H2 masses. We
present gas scaling relations, gas fractions, and mass functions from z = 0 to
z = 3 and the gas properties of galaxies as a function of their host halo
masses. Predictions of our work include: 1) there is a ~ 0.2 dex decrease in
the H I mass of galaxies as a function of their stellar mass since z = 1.5,
whereas the H2 mass of galaxies decreases by > 1 dex over the same period. 2)
galaxy cold gas fractions and H2 fractions decrease with increasing stellar
mass and time. Galaxies with M* > 10^10 Msun are dominated by their stellar
content at z < 1, whereas less-massive galaxies only reach these gas fractions
at z = 0. We find the strongest evolution in relative gas content at z < 1.5.
3) the SFR to gas mass ratio decreases by an order of magnitude from z = 3 to z
= 0. This is consistent with lower H2 fractions; these lower fractions in
combination with smaller gas reservoirs correspond to decreased present-day
galaxy SFRs. 4) an H2-based star- formation relation can simultaneously fuel
the evolution of the cosmic star-formation and reproduce the observed weak
evolution in the cosmic HI density. 5) galaxies residing in haloes with masses
near 10^12 Msun are most efficient at obtaining large gas reservoirs and
forming H2 at all redshifts. These two effects lie at the origin of the high
star-formation efficiencies in haloes with the same mass.Comment: accepted for publication in MNRAS, 20 pages, 16 figures (+ 1 figure
in appendix), data files are accessible through
http://www.eso.org/~gpopping/Gergo_Poppings_Homepage/Data.htm
The nature of the ISM in galaxies during the star-formation activity peak of the Universe
We combine a semi-analytic model of galaxy formation, tracking atomic and
molecular phases of cold gas, with a three-dimensional radiative-transfer and
line tracing code to study the sub-mm emission from atomic and molecular
species (CO, HCN, [CI], [CII], [OI]) in galaxies. We compare the physics that
drives the formation of stars at the epoch of peak star formation (SF) in the
Universe (z = 2.0) with that in local galaxies. We find that normal
star-forming galaxies at high redshift have much higher CO-excitation peaks
than their local counterparts and that CO cooling takes place at higher
excitation levels. CO line ratios increase with redshift as a function of
galaxy star-formation rate, but are well correlated with H2 surface density
independent of redshift. We find an increase in the [OI]/[CII] line ratio in
typical star-forming galaxies at z = 1.2 and z = 2.0 with respect to
counterparts at z = 0. Our model results suggest that typical star-forming
galaxies at high redshift consist of much denser and warmer star-forming clouds
than their local counterparts. Galaxies belonging to the tail of the SF
activity peak at z = 1.2 are already less dense and cooler than counterparts
during the actual peak of SF activity (z = 2.0). We use our results to discuss
how future ALMA surveys can best confront our predictions and constrain models
of galaxy formation.Comment: 19 pages, 14 figures, accepted for publication in MNRA
Hierarchical Bayesian inference of the Initial Mass Function in Composite Stellar Populations
The initial mass function (IMF) is a key ingredient in many studies of galaxy
formation and evolution. Although the IMF is often assumed to be universal,
there is continuing evidence that it is not universal. Spectroscopic studies
that derive the IMF of the unresolved stellar populations of a galaxy often
assume that this spectrum can be described by a single stellar population
(SSP). To alleviate these limitations, in this paper we have developed a unique
hierarchical Bayesian framework for modelling composite stellar populations
(CSPs). Within this framework we use a parameterized IMF prior to regulate a
direct inference of the IMF. We use this new framework to determine the number
of SSPs that is required to fit a set of realistic CSP mock spectra. The CSP
mock spectra that we use are based on semi-analytic models and have an IMF that
varies as a function of stellar velocity dispersion of the galaxy. Our results
suggest that using a single SSP biases the determination of the IMF slope to a
higher value than the true slope, although the trend with stellar velocity
dispersion is overall recovered. If we include more SSPs in the fit, the
Bayesian evidence increases significantly and the inferred IMF slopes of our
mock spectra converge, within the errors, to their true values. Most of the
bias is already removed by using two SSPs instead of one. We show that we can
reconstruct the variable IMF of our mock spectra for signal-to-noise ratios
exceeding 75.Comment: Accepted for publication in MNRAS, 16 pages, 8 figure
Automated mining of the ALMA archive in the COSMOS field (A3COSMOS): II. Cold molecular gas evolution out to Redshift 6
We present new measurements of the cosmic cold molecular gas evolution out to redshift 6 based on systematic mining of the ALMA public archive in the COSMOS deep field (A3COSMOS). Our A3COSMOS dataset contains ~700 galaxies (0.3 < z < 6) with high-confidence ALMA detections in the (sub-)millimeter continuum and multi-wavelength spectral energy distributions (SEDs). Multiple gas mass calibration methods are compared and biases in band conversions (from observed ALMA wavelength to rest-frame Rayleigh-Jeans(RJ)-tail continuum) have been tested. Combining our A3COSMOS sample with ~1,000 CO-observed galaxies at 0 < z < 4 (75% at z < 0.1), we parameterize galaxies' molecular gas depletion time and molecular gas to stellar mass ratio (gas fraction) each as a function of the stellar mass, offset from the star-forming main sequence (Delta MS) and cosmic age (or redshift). Our proposed functional form provides a statistically better fit to current data (than functional forms in the literature), and implies a "downsizing" effect (i.e., more-massive galaxies evolve earlier than less-massive ones) and "mass-quenching" (gas consumption slows down with cosmic time for massive galaxies but speeds up for low-mass ones). Adopting galaxy stellar mass functions and applying our function for gas mass calculation, we for the first time infer the cosmic cold molecular gas density evolution out to redshift 6 and find agreement with CO blind surveys as well as semi-analytic modeling. These together provide a coherent picture of cold molecular gas, SFR and stellar mass evolution in galaxies across cosmic time
Tracing Molecular Gas Mass in z â 6 Galaxies with [C ii]
We investigate the fine-structure [C] line at
m as a molecular gas tracer by analyzing the relationship between
molecular gas mass () and [C] line
luminosity () in 11,125 star-forming, main sequence
galaxies from the SIMBA simulations, with line emission modeled by S\'IGAME.
Though most () of the gas mass in our simulations is ionized,
the bulk () of the [C] emission comes from the
molecular phase. We find a sub-linear (slope ) relation, in contrast with the linear relation derived
from observational samples of more massive, metal-rich galaxies at . We derive a median [C]-to- conversion
factor of .
This is lower than the average value of derived from observations, which we attribute to lower gas-phase
metallicities in our simulations. Thus, a lower, luminosity-dependent,
conversion factor must be applied when inferring molecular gas masses from
[C] observations of low-mass galaxies. For our
simulations, [C] is a better tracer of the molecular gas
than CO , especially at the lowest metallicities, where much of the gas
is 'CO-dark'. We find that is more tightly correlated with
than with star-formation rate (), and both the and
relations arise from the Kennicutt-Schmidt relation. Our findings suggest that
is a promising tracer of the molecular gas at the earliest
cosmic epochs.Comment: 13 pages, 9 figures. Accepted for publication in Ap
An indirect measurement of gas evolution in galaxies at 0.5 < z < 2.0
One key piece of information missing from high-redshift galaxy surveys is the galaxies cold gas contents. We present a new method to indirectly determine cold gas surface densities and integrated gas masses from galaxy star formation rates and to separate the atomic and molecular gas components. Our predicted molecular and total gas surface densities and integrated masses are in very good agreement with direct measurements quoted in the literature for low- and high-z galaxies. We apply this method to predict the gas content for a sample of similar to 57?000 galaxies in the Cosmic Evolution Survey (COSMOS) field at 0.5 <z <2.0, selected to have IAB <24?mag. This approach allows us to investigate in detail the redshift evolution of galaxy's cold and molecular gas content versus stellar mass and to provide fitting formulae for galaxy gas fractions. We find a clear trend between galaxy gas fraction, molecular gas fraction and stellar mass with redshift, suggesting that massive galaxies consume and/or expel their gas at higher redshift than less massive objects and have lower fractions of their gas in molecular form. The characteristic stellar mass separating gas- from stellar-dominated galaxies decreases with time. This indicates that massive galaxies reach a gas-poor state earlier than less massive objects. These trends can be considered to be another manifestation of downsizing in star formation activity
Faint [CI](1-0) emission in z 3.5 radio galaxies
We present Atacama Large Millimeter/sub-millimeter Array (ALMA) neutral
carbon, [C I](1-0), line observations that probe molecular hydrogen gas (H)
within seven radio galaxies at surrounded by extended
( kpc) Ly- nebulae. We extract [C I](1-0) emission from the
radio-active galactic nuclei (AGN) host galaxies whose positions are set by
near-infrared detections and radio detections of the cores. Additionally, we
place constraints on the galaxies' systemic redshifts via He II 1640
lines seen with the Multi-Unit Spectroscopic Explorer (MUSE). We detect faint
[C I] emission in four out of seven sources. In two of these galaxies, we
discover narrow line emission of full width at half maximum km
s which may trace emission from bright kpc-scale gas clouds within the
ISM. In the other two [C I]-detected galaxies, line dispersions range from
km s and may be tracing the rotational component of the
cold gas. Overall, the [C I] line luminosities correspond to H masses of
M for the detections
and M for the [C I] non-detections
in three out of seven galaxies within the sample. The molecular gas masses in
our sample are relatively low in comparison to previously reported measures for
similar galaxies which are M Our
results imply that the observed faintness in carbon emission is representative
of a decline in molecular gas supply from previous star-formation epochs and/or
a displacement of molecular gas from the ISM due to jet-powered outflows.Comment: 16 pages, 4 figures and 5 tables. Accepted for publication in MNRA
Cold gas outflows from the Small Magellanic Cloud traced with ASKAP
Feedback from massive stars plays a critical role in the evolution of the
Universe by driving powerful outflows from galaxies that enrich the
intergalactic medium and regulate star formation. An important source of
outflows may be the most numerous galaxies in the Universe: dwarf galaxies.
With small gravitational potential wells, these galaxies easily lose their
star-forming material in the presence of intense stellar feedback. Here, we
show that the nearby dwarf galaxy, the Small Magellanic Cloud (SMC), has atomic
hydrogen outflows extending at least 2 kiloparsecs (kpc) from the star-forming
bar of the galaxy. The outflows are cold, , and may have formed
during a period of active star formation million years (Myr) ago. The
total mass of atomic gas in the outflow is solar masses, , or % of the total atomic gas of the galaxy. The inferred
mass flux in atomic gas alone, , is up to an order of magnitude greater than the star
formation rate. We suggest that most of the observed outflow will be stripped
from the SMC through its interaction with its companion, the Large Magellanic
Cloud (LMC), and the Milky Way, feeding the Magellanic Stream of hydrogen
encircling the Milky Way.Comment: Published in Nature Astronomy, 29 October 2018,
http://dx.doi.org/10.1038/s41550-018-0608-
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