21-cm absorbers at intermediate redshifts
  Damped Lyman-alpha systems (DLAs) seen in the spectra of high-z QSOs allow us
to probe the physical conditions in protogalaxies. Our understanding of
physical conditions in DLAs at high-z is primarily based on the absorption
lines of H_2 molecules and fine-structure transitions. Another important way of
probing the thermal state of interstellar medium in these systems is by
studying the 21-cm absorption in the spectra of background quasars. Here we
report the main results of our GMRT survey to search for 21-cm absorption in a
representative and unbiased sample of 35 DLA candidates at 1.10<z<1.45. Our
sample of DLA candidates is drawn from the strong MgII systems in SDSS DR5 and
has resulted in discovery of 9 new 21-cm absorbers. Prior to our survey only
one 21-cm absorber was known in the redshift range: 0.7<z<2. This survey has
allowed us to investigate the dependence of detectability of 21-cm absorption
on the properties of UV absorption lines detected in SDSS spectra and estimate
the number per unit redshift of 21-cm absorbers. Our GMRT survey provides a
representative sample of systems that can be used in combination with various
follow-up observations: (1) for investigating the physical conditions in the
absorbing gas using spin temperature measurements, (2) for investigating the
effect of metallicity and dust content on the detectability of 21-cm
absorption, (3) for studying the morphology of the absorbing gas and (4) for
probing the time evolution of various fundamental constants. Results from the
first phase of our survey are presented in Gupta et al. (2007). Detailed
description of the entire sample and results from the survey are presented in
Gupta et al. (2009).
