HI 21-cm absorption survey of quasar-galaxy pairs: Distribution of cold
  gas around z<0.4 galaxies
  We present the results from our survey of HI 21-cm absorption, using GMRT,
VLA and WSRT, in a sample of 55 z<0.4 galaxies towards radio sources with
impact parameters (b) in the range ~0-35 kpc. In our primary sample (defined
for statistical analyses) of 40 quasar-galaxy-pairs (QGPs), probed by 45
sightlines, we have found seven HI 21-cm absorption detections, two of which
are reported here for the first time. Combining our primary sample with
measurements having similar optical depth sensitivity ($\int\tau dv$ <= 0.3
km/s) from the literature, we find a weak anti-correlation (rank correlation
coefficient = -0.20 at 2.42sigma level) between $\int\tau dv$ and b, consistent
with previous literature results. The covering factor of HI 21-cm absorbers
(C_21) is estimated to be 0.24 (+0.12/-0.08) at b <= 15 kpc and 0.06
(+0.09/-0.04) at b = 15-35 kpc. $\int\tau dv$ and C_21 show similar declining
trend with radial distance along the galaxy's major axis and distances scaled
with the effective HI radius. There is also tentative indication that most of
the HI 21-cm absorbers could be co-planar with the extended HI discs. No
significant dependence of $\int\tau dv$ and C_21 on galaxy luminosity, stellar
mass, colour and star formation rate is found, though the HI 21-cm absorbing
gas cross-section may be larger for the luminous galaxies. The higher detection
rate (by a factor of ~4) of HI 21-cm absorption in z<1 DLAs compared to the
QGPs indicates towards small covering factor and patchy distribution of cold
gas clouds around low-z galaxies.
