Atomic carbon as a powerful tracer of molecular gas in the high-redshift
  Universe: perspectives for ALMA
  We use a high-resolution hydrodynamic simulation that tracks the
non-equilibrium abundance of molecular hydrogen within a massive high-redshift
galaxy to produce mock Atacama Large Millimeter Array (ALMA) maps of the
fine-structure lines of atomic carbon, CI 1-0 and CI 2-1. Inspired by recent
observational and theoretical work, we assume that CI is thoroughly mixed
within giant molecular clouds and demonstrate that its emission is an excellent
proxy for H2. Nearly all of the H2 associated with the galaxy can be detected
at redshifts z<4 using a compact interferometric configuration with a large
synthesized beam (that does not resolve the target galaxy) in less than 4 h of
integration time. Low-resolution imaging of the \CI lines (in which the target
galaxy is resolved into three to four beams) will detect ~80 per cent of the H2
in less than 12 h of aperture synthesis. In this case, the resulting data cube
also provides the crucial information necessary for determining the dynamical
state of the galaxy. We conclude that ALMA observations of the CI 1-0 and 2-1
emission are well-suited for extending the interval of cosmic look-back time
over which the H2 distributions, the dynamical masses, and the Tully-Fisher
relation of galaxies can be robustly probed.
