Over the last decade, observations of the cool ISM in distant galaxies via molecular line and atomic fine structure line emission has gone from a curious look into a few extreme, rare objects, to a mainstream tool in the study of galaxy formation, out to the highest redshifts (z ∼ 7). Molecular gas has now been observed in close to 200 galaxies, including numerous AGN host–galaxies, extreme starburst SMGs, and increasing samples of ‘main–sequence' CSG. Studies have moved well beyond simple detection, to dynamical imaging at kpc–scale resolution, and multi–line, multi–species studies of the ISM in early galaxies. Study of atomic fine structure line emission is also rapidly accelerating, with some tens of galaxies detected in [C II] 158 µm, and other species, at z > 1, including detection of the most distant quasar with a spectroscopic redshift (z = 7.08).
The results of these studies are extremely telling for models of galaxy formation, providing the required complement to studies of the stars and star formation in early galaxies. One of the most exciting empirical result is the discovery that CSG have CO luminosities approaching those of SMGs and quasar hosts, but FIR luminosities close to an order of magnitude less. The higher space density of the CSG galaxies provides a rich hunting ground for molecular line studies of distant galaxies. Observations suggest that the gas fraction (Mgas / Mstars) in massive disk galaxies increases by an order of magnitude from z ∼ 0 to z ≥ 1.5. Hence, the epoch of peak cosmic star formation density corresponds to an epoch of gas–dominated disks.
For the rarer, hyper–starburst galaxies, the quasar hosts and powerful radio galaxies show the most extreme gas properties, in terms of gas excitation, star formation ‘efficiency', and compact although complex, gas morphologies. These results indicate compact, hyper–starbursts coeval with Eddington–limited AGN accretion. Submm galaxies are a mixed bag of gas rich mergers and extended, gas rich disks in dense cosmic environments.
Current measurements suggest that the hyper–starbursts have a low CO luminosity to gas mass conversion factor, α ∼ 0.8, consistent with the extreme dense ISM conditions seen in nearby nuclear starbursts. The CSG are consistent with a Milky Way GMC value of α ∼ 4. There is increasing evidence that α increases with decreasing metallicity in galaxies, and in general, there may be a continuum of values of α, depending on ISM pressure, dynamics, and metallicity. The correlation between CO and FIR luminosity suggests two populations: starburst galaxies with rapid gas consumption timescales of a few × 107 years, and main sequence galaxies with gas consumption timescales an order of magnitude longer.
The strong ISM gas cooling line from [C II] is proving to be a key tool in the study of the dynamics of the earliest galaxies. [C II] imaging of z > 4 galaxies has already revealed a ‘maximal starburst disk' on sub–kpc scales, likely rotating disks on few to 10 kpc–scales, and possible tidal structures on even larger scales. ALMA has already demonstrated the ability to detect [C II] emission from LAEs and LBGs at high redshift.
We have made a first attempt at quantifying the dense gas history of the Universe, based on current observations. While admittedly gross, these measurements are consistent with modeling based on large scale cosmological simulations, and on empirical models based on assumed star formation laws.