Science with an ngVLA. Cold gas in High-z Galaxies: The dense ISM
  The goal of this science case 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 and/or at too low
frequencies to be effectively observed in high redshift galaxies. The ngVLA
will offer for the first time the sensitivity at radio frequencies that is
needed to target [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 various redshifts
$z>1$), beyond the tip of the iceberg of the hyper-luminous sources that could
be studied up to 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.
