Cold neutral hydrogen gas in galaxies
  This review summarizes recent studies of the cold neutral hydrogen gas
associated with galaxies probed via the HI 21-cm absorption line. HI 21-cm
absorption against background radio-loud quasars is a powerful tool to study
the neutral gas distribution and kinematics in foreground galaxies from
kilo-parsec to parsec scales. At low redshifts (z<0.4), it has been used to
characterize the distribution of high column density neutral gas around
galaxies and study the connection of this gas with the galaxy's optical
properties. The neutral gas around galaxies has been found to be patchy in
distribution, with variations in optical depth observed at both kilo-parsec and
parsec scales. At high redshifts (z>0.5), HI 21-cm absorption has been used to
study the neutral gas in metal or Lyman-alpha absorption-selected galaxies. It
has been found to be closely linked with the metal and dust content of the gas.
Trends of various properties like incidence, spin temperature and velocity
width of HI 21-cm absorption with redshift have been studied, which imply
evolution of cold gas properties in galaxies with cosmic time. Upcoming large
blind surveys of HI 21-cm absorption with next generation radio telescopes are
expected to determine accurately the redshift evolution of the number density
of HI 21-cm absorbers per unit redshift and hence understand what drives the
global star formation rate density evolution.
