Radio studies of galaxy formation: Dense Gas History of the Universe
  Line and continuum studies at centimeter through submillimeter wavelengths
address probe deep into the earliest, most active and dust obscured phases of
galaxy formation, and reveal the molecular and cool atomic gas. We summarize
the techniques of radio astronomy to perform these studies, then review the
progress on radio studies of galaxy formation. The dominant work over the last
decade has focused on massive, luminous starburst galaxies (submm galaxies and
AGN host galaxies). The far infrared luminosities are ~ 1e13 Lsun, implying
star formation rates, SFR > 1e3 Msun/year. Molecular gas reservoirs are found
with masses: M(H_2) > 1e10 (alpha/0.8}) Msun. The CO excitation in these
luminous systems is much higher than in low redshift spiral galaxies. Imaging
of the gas distribution and dynamics suggests strongly interacting and merging
galaxies, indicating gravitationally induced, short duration (~ 1e7 year)
starbursts. These systems correspond to a major star formation episode in
massive galaxies in proto-clusters at intermediate to high redshift. Recently,
radio observations have probed the more typical star forming galaxy population
(SFR ~ 100 Msun/year), during the peak epoch of Universal star formation (z ~
1.5 to 2.5). These observations reveal massive gas reservoirs without
hyper-starbursts, and show that active star formation occurs over a wide range
in galaxy stellar mass. The conditions in this gas are comparable to those
found in the Milky Way disk. A key result is that the peak epoch of star
formation in the Universe also corresponds to an epoch when the baryon content
of star forming galaxies was dominated by molecular gas, not stars. We consider
the possibility of tracing out the dense gas history of the Universe, and
perform initial, admittedly gross, calculations. ABRIDGED
