Abstract
We use a high-resolution simulation that tracks the non-equilibrium abundance
of molecular hydrogen, H2, within a massive high-redshift galaxy to produce
mock 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 in giant molecular clouds and demonstrate that its emission is
an excellent proxy for H2. The entire H2 mass of a galaxy at redshift z<4 can
be detected using a compact interferometric configuration with a large
synthesized beam (that does not resolve the target galaxy) in less than 1 hour
of integration time. Low-resolution imaging of the CI lines (in which the
target galaxy is resolved into 3-4 beams) will detect nearly 50-60 per cent of
the molecular hydrogen in less than 12 hours. In this case, the data cube also
provides valuable information regarding the dynamical state of the galaxy. We
conclude that ALMA observations of the CI 1-0 and 2-1 emission will widely
extend the interval of cosmic look-back time over which the H2 gas mass
distribution, the dynamical masses, and the Tully-Fisher relation of galaxies
can be accurately probed.
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