Cool Gas in High-Redshift Galaxies

Abstract

Over the past decade, observations of the cool interstellar medium (ISM) in distant galaxies via molecular and atomic fine structure line (FSL) emission have gone from a curious look into a few extreme, rare objects to a mainstream tool for studying galaxy formation out to the highest redshifts. Molecular gas has been observed in close to 200 galaxies at z > 1, including numerous AGN host-galaxies out to z ∼ 7, highly star-forming submillimeter galaxies, and increasing samples of main-sequence color-selected star-forming galaxies at z ∼ 1.5 to 2.5. Studies have moved well beyond simple detections to dynamical imaging at kiloparsec-scale resolution and multiline, multispecies studies that determine the physical conditions in the ISM in early galaxies. Observations of the cool gas are the required complement to studies of the stellar density and star-formation history of the Universe as they reveal the phase of the ISM that immediately precedes star formation in galaxies. Current observations suggest that the order of magnitude increase in the cosmic star-formation rate density from z ∼ 0 to 2 is commensurate with a similar increase in the gas-to-stellar mass ratio in star-forming disk galaxies. Progress has been made in determining the CO luminosity to H2 mass conversion factor at high z, and the dichotomy between high versus low values for main-sequence versus starburst galaxies, respectively, appears to persist with increasing redshift, with a likely dependence on metallicity and other local physical conditions. There may also be two sequences in the relationship between star-formation rate and gas mass: one for starbursts, in which the gas consumption timescale is short (a few 107 years), and one for main sequence galaxies, with an order of magnitude longer gas consumption timescale. Studies of atomic FSL emission are rapidly progressing, with some tens of galaxies detected in the exceptionally bright [Cii] 158-μm line to date. The [Cii] line is proving to be a unique tracer of galaxy dynamics in the early Universe and, together with other atomic FSLs, has the potential to be the most direct means of obtaining spectroscopic redshifts for the first galaxies during cosmic reionization.