A census on the gas content of star-forming galaxies over the past 11 billion years of the universe

Abstract

With the advent of the Atacama Large Millimeter/submillimeter Array (ALMA), we now have the opportunity to measure the physical properties of the cold, dense, star-forming interstellar medium (ISM) even in distant, high-redshift galaxies. This dissertation documents measurements of the mean molecular gas mass, <em>M</em>_gas, and apparent size of star-forming galaxies (SFGs) at different cosmological epochs, for galaxies over a wide range of total stellar mass, <em>M</em>_&#9733;, and in different star formation modes, normal vs. starburst. This contributes to constrain the physical mechanisms that regulate star formation throughout cosmic history. <br /> It has been known that most of the SFGs follow a tight correlation between total star formation rate and total stellar mass, the so-called main sequence (MS) of SFGs. I selected a sample of mass-complete MS galaxies and measured their mean <em>M</em>_gas and size using a Fourier-domain stacking analysis. Based on the measurements, I determined the mean molecular gas fraction, <em>M</em>_gas/<em>M</em>_&#9733;, and the mean molecular gas depletion time, <em>M</em>_gas/ star formation rate (SFR), of these SFGs. I found that MS galaxies exhibit a constant mean molecular gas depletion time of &#126;400 million years over cosmic time, while their mean molecular gas fraction decreases by a factor of &#126;24 over the past 11 billion years. These findings suggest that cold gas must be continuously accreted to MS galaxies from the cosmic web to sustain star formation on timescales of giga-years, and that the evolution of MS galaxies is predominantly regulated by variations in their gas content. The mean gas sizes of MS galaxies remain constant (&#126;2.2 kpc) across all stellar masses and throughout cosmic time, {which is smaller than the mean size of their monochromatic optical emissions, but similar to the mean size of their stellar mass distributions.} This suggests negative radial gradient of the dust attenuation in MS galaxies. <br /> By integrating the gas mass, size, and star formation rate of MS galaxies, I investigated the relation between their gas mass surface density and the SFR surface density, known as the Kennicutt-Schmidt relation. An universal slope of &#126;1.13 in log-space across cosmic time is found, which implies that the increase in mean molecular gas depletion time of observed over the past 11~Gyr is due to a decrease in the gas surface density. <br /> On the origin of a &#126;0.3 dex scatter in the SFR on the MS at a given redshift and <em>M</em>_&#9733;, I found that SFGs located on the lower envelop of the MS have lower molecular gas fraction and longer depletion time, while SFGs located on the upper envelope have higher gas mass fractions and shorter depletion time. This suggests that small variations in the SFR of SFGs during their long-term evolution within the MS are regulated by disturbances acting on short timescales, such as minor mergers and violent disk instabilities. <br /> At any redshift and total stellar mass, a subset of SFGs have star formation rates of more than a factor of three higher than those of MS galaxies. These are the so-called starburst galaxies, which I have found to exhibit complex optical morphologies indicative of merger events that appear as infall of gas into deeper gravitational potentials, as evidenced by more compact gas sizes (&#126;1.4 kpc). This infalling gas scenario is supported by my finding that starburst galaxies have larger molecular gas fractions than MS galaxies, because neutral hydrogen gas would lose its angular momentum and fall into the deeper gravitational potentials, transforming into a molecular phase.