Cold Gas in High-z Galaxies: The Dense ISM

The goal of this chapter is to study physical conditions of the interstellar medium (ISM) in distant galaxies. In particular, its densest component is associated with the inner cores of clouds - this is where star formation takes place. Carbon monoxide is usually used to trace molecular gas emission; however, its transitions are practically opaque, thus preventing astronomers from piercing through the clouds, into the deepest layers that are most intimately connected with the formation of stars. Other dense gas tracers are required, although they are typically too faint to be effectively observed in high redshift main sequence galaxies. The ngVLA will offer for the first time the sensitivity at radio frequencies that is needed to detect [CI]1-0 at z ≥ 5, as well as the ground transitions of dense gas tracers of the ISM such as HCN, HNC, HCO+ at z > 1, beyond the tip of the iceberg of the hyper-luminous sources that can be studied now. These new tools will critically contribute to our understanding of the intimate interplay between gas clouds and star formation in different environments and cosmic epochs