Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe evolution of the cosmic molecular gas density
120
0
0.0
(
0
)
Authors
Fabian Walter
Ask ChatGPT about the research
No Arabic abstract
One of the last missing pieces in the puzzle of galaxy formation and evolution through cosmic history is a detailed picture of the role of the cold gas supply in the star-formation process. Cold gas is the fuel for star formation, and thus regulates the buildup of stellar mass, both through the amount of material present through a galaxys gas mass fraction, and through the efficiency at which it is converted to stars. Over the last decade, important progress has been made in understanding the relative importance of these two factors along with the role of feedback, and the first measurements of the volume density of cold gas out to redshift 4, (the cold gas history of the Universe) has been obtained. To match the precision of measurements of the star formation and black-hole accretion histories over the coming decades, a two orders of magnitude improvement in molecular line survey speeds is required compared to what is possible with current facilities. Possible pathways towards such large gains include significant upgrades to current facilities like ALMA by 2030 (and beyond), and eventually the construction of a new generation of radio-to-millimeter wavelength facilities, such as the next generation Very Large Array (ngVLA) concept.
rate research
Read More
We present new empirical constraints on the evolution of $rho_{rm H_2}$, the cosmological mass density of molecular hydrogen, back to $zapprox2.5$. We employ a statistical approach measuring the average observed $850mu{rm m}$ flux density of near-infrared selected galaxies as a function of redshift. The redshift range considered corresponds to a span where the $850mu{rm m}$ band probes the Rayleigh-Jeans tail of thermal dust emission in the rest-frame, and can therefore be used as an estimate of the mass of the interstellar medium (ISM). Our sample comprises of ${approx}150,000$ galaxies in the UKIDSS-UDS field with near-infrared magnitudes $K_{rm AB}leq25$ mag and photometric redshifts with corresponding probability distribution functions derived from deep 12-band photometry. With a sample approximately 2 orders of magnitude larger than in previous works we significantly reduce statistical uncertainties on $rho_{rm H_2}$ to $zapprox2.5$. Our measurements are in broad agreement with recent direct estimates from blank field molecular gas surveys, finding that the epoch of molecular gas coincides with the peak epoch of star formation with $rho_{rm H_2}approx2times10^7,{rm M_odot},{rm Mpc^{-3}}$ at $zapprox2$. We demonstrate that $rho_{rm H_2}$ can be broadly modelled by inverting the star-formation rate density with a fixed or weakly evolving star-formation efficiency. This constant efficiency model shows a similar evolution to our statistically derived $rho_{rm H_2}$, indicating that the dominant factor driving the peak star formation history at $zapprox2$ is a larger supply of molecular gas in galaxies rather than a significant evolution of the star-formation rate efficiency within individual galaxies.
In this paper we use ASPECS, the ALMA Spectroscopic Survey in the {em Hubble} Ultra Deep Field (UDF) in band 3 and band 6, to place blind constraints on the CO luminosity function and the evolution of the cosmic molecular gas density as a function of redshift up to $zsim 4.5$. This study is based on galaxies that have been solely selected through their CO emission and not through any other property. In all of the redshift bins the ASPECS measurements reach the predicted `knee of the CO luminosity function (around $5times10^{9}$ K km/s pc$^2$). We find clear evidence of an evolution in the CO luminosity function with respect to $zsim 0$, with more CO luminous galaxies present at $zsim 2$. The observed galaxies at $zsim 2$ also appear more gas-rich than predicted by recent semi-analytical models. The comoving cosmic molecular gas density within galaxies as a function of redshift shows a factor 3-10 drop from $z sim 2$ to $z sim 0$ (with significant error bars), and possibly a decline at $z>3$. This trend is similar to the observed evolution of the cosmic star formation rate density. The latter therefore appears to be at least partly driven by the increased availability of molecular gas reservoirs at the peak of cosmic star formation ($zsim2$).
We study the evolution of the cold gas content of galaxies by splitting the interstellar medium into its atomic and molecular hydrogen components, using the galaxy formation model GALFORM in the LCDM framework. We calculate the molecular-to-atomic hydrogen mass ratio, H2/HI, in each galaxy using two different approaches; the pressure-based empirical relation of Blitz & Rosolowsky and the theoretical model of Krumholz, McKeee & Tumlinson, and apply them to consistently calculate the star formation rates of galaxies. We find that the model based on the Blitz & Rosolowsky law predicts an HI mass function, CO(1-0) luminosity function, correlations between the H2/HI ratio and stellar and cold gas mass, and infrared-CO luminosity relation in good agreement with local and high redshift observations. The HI mass function evolves weakly with redshift, with the number density of high mass galaxies decreasing with increasing redshift. In the case of the H2 mass function, the number density of massive galaxies increases strongly from z=0 to z=2, followed by weak evolution up to z=4. We also find that the H2/HI ratio of galaxies is strongly dependent on stellar and cold gas mass, and also on redshift. The slopes of the correlations between H2/HI and stellar and cold gas mass hardly evolve, but the normalisation increases by up to two orders of magnitude from z=0-8. The strong evolution in the H2 mass function and the H2/HI ratio is primarily due to the evolution in the sizes of galaxies and secondarily, in the gas fractions. The predicted cosmic density evolution of HI agrees with the observed evolution inferred from DLAs, and is dominated by low/intermediate mass halos. We find that previous theoretical studies have largely overestimated the redshift evolution of the global H2/HI ratio due to limited resolution. We predict a maximum of rho_H2/rho_HI~1.2 at z~3.5.
We surveyed the circumnuclear disk of the Seyfert galaxy NGC1068 between the frequencies 86.2 GHz and 115.6 GHz, and identified 17 different molecules. Using a time and depth dependent chemical model we reproduced the observational results, and show that the column densities of most of the species are better reproduced if the molecular gas is heavily pervaded by a high cosmic ray ionization rate of about 1000 times that of the Milky Way. We discuss how molecules in the NGC1068 nucleus may be influenced by this external radiation, as well as by UV radiation fields.
In our grid of multiphase chemical evolution models (Molla & Diaz, 2005), star formation in the disk occurs in two steps: first, molecular gas forms, and then stars are created by cloud-cloud collisions or interactions of massive stars with the surrounding molecular clouds. The formation of both molecular clouds and stars are treated through the use of free parameters we refer to as efficiencies. In this work we modify the formation of molecular clouds based on several new prescriptions existing in the literature, and we compare the results obtained for a chemical evolution model of the Milky Way Galaxy regarding the evolution of the Solar region, the radial structure of the Galactic disk, and the ratio between the diffuse and molecular components, HI/H$_2$. Our results show that the six prescriptions we have tested reproduce fairly consistent most of the observed trends, differing mostly in their predictions for the (poorly-constrained) outskirts of the Milky Way and the evolution in time of its radial structure. Among them, the model proposed by Ascasibar et al. (2017), where the conversion of diffuse gas into molecular clouds depends on the local stellar and gas densities as well as on the gas metallicity, seems to provide the best overall match to the observed data.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
