Physical Conditions in the Interstellar Medium of High-Redshift
  Galaxies: Mass Budget and Gas Excitation
  Following the first pioneering efforts in the 1990s that have focused on the
detection of the molecular interstellar medium in high redshift galaxies,
recent years have brought great advances in our understanding of the actual
physical properties of the gas that set the conditions for star formation.
Observations of the ground-state CO J=1-0 line have furnished crucial
information on the total masses of the gas reservoirs, as well as reliable
dynamical mass and size estimates. Detailed studies of rotational ladders of CO
have provided insight on the temperature and density of the gas. Investigations
of the very dense gas associated with actively star-forming regions in the
interstellar medium, most prominently through HCN and HCO+, have enabled a
better understanding of the nature of the extreme starbursts found in many
high-redshift galaxies, which exceed the star formation rates of their most
active present-day counterparts by an order of magnitude. Key progress in this
area has been made through targeted studies of few, well-selected systems with
current facilities. With the completion of the Karl G. Jansky Very Large Array
and the Atacama Large (sub)Millimeter Array, it will become possible to develop
a more general framework for the interpretation of these investigations based
on unbiased studies of "normal" star-forming galaxies back to the earliest
cosmic epochs.
