Cool Gas in High Redshift Galaxies
  Over the last decade, observations of the cool interstellar medium in distant
galaxies via molecular and atomic fine structure line emission has gone from a
curious look into a few extreme, rare objects, to a mainstream tool to study
galaxy formation, out to the highest redshifts. Molecular gas has now been
observed in close to 200 galaxies at z>1, including numerous AGN host-galaxies
out to z~7, highly starforming sub-millimeter galaxies (median redshift z~2.5),
and increasing samples of 'main-sequence' star forming galaxies at z~1.5-2.5.
Studies have moved well beyond simple detections, to dynamical imaging at
kpc-scale resolution, and multi-line, multi-species studies that determine the
physical conditions in the interstellar medium. Observations of the cool gas
are the required complement to studies of the stellar density and star
formation history of the Universe, as they reveal the phase of the interstellar
medium that immediately precedes star formation. Current observations suggest
that the order of magnitude increase in the cosmic star formation rate density
from z~0 to 2 is commensurate with a similar increase in the gas to stellar
mass ratio in star forming disk galaxies. Progress has been made on determining
the CO luminosity to H_2 mass conversion factor at high-z, and the dichotomy
between high versus low depletion time values for main sequence versus
starburst galaxies, respectively, with a likely dependence on metallicity and
other local physical conditions. Studies of atomic fine structure line emission
are rapidly progressing, with some tens of galaxies detected in the
exceptionally bright [CII] 158 micron line to date. This line is proving to be
a unique tracer of galaxy dynamics in the early Universe and has the potential
to be the most direct means of obtaining spectroscopic redshifts for the first
galaxies during cosmic reionization.
