The Molecular ISM of Quasar Host Galaxies in the Early Universe

Abstract

Detailed studies of the molecular gas phase in the host galaxies of the highest redshift quasars are important for our understanding of the formation and evolution of quasars and their bulges, since the molecular gas is the prerequisite material for star formation. This investigation capitalizes on state-of-the-art observations in the radio/millimeter wavelength regime to study the key properties of the molecular interstellar medium in some of the most distant, gas-rich quasars. To search for evolutionary, luminosity-dependent, or galaxy type-dependent trends in the conditions under which star formation takes place, results are interpreted in the context of studies of nearby galaxies and high redshift galaxy populations. From the first high-resolution CO(J=1→0) spectroscopy of high-z quasars, the total molecular gas mass of their host galaxies is determined. By more than doubling the number of molecules known in the distant universe [from 2 (CO/HCN) to 5], it is found that multiple molecular probes of dense gas predict similar star formation rates within the dense molecular regions of high redshift galaxies, out to the first 2Gyr after the Big Bang. Together with other studies, these results indicate an increase in star formation efficiency toward the most luminous distant gas-rich systems, possibly due to a higher median gas density. In a connected, time consuming interferometric study, the host galaxies of three z>4 quasars are resolved, for the first time, both spatially (at up to 0.15, or 1.0 kpc) and in velocity space, revealing that the molecular reservoirs show a wealth of morphologies. The derived dynamical masses are large enough to account for both the central supermassive black holes and the full reservoirs of molecular gas, but do not leave much room for a stellar bulge as predicted by the local relation between black hole mass and bulge velocity dispersion. Quasar host galaxies are thus prime laboratories to study the coevolution of supermassive black holes and the extreme starbursts hosted by the molecular environments at early cosmic epochs. The observations presented here thus provide an important foundation for future studies of normal galaxies at high redshifts with the Atacama Large Millimeter Array (ALMA).