GMRT mini-survey to search for 21-cm absorption in Quasar-Galaxy Pairs
  at z~0.1
  We present the results from our 21-cm absorption survey of a sample of 5
quasar-galaxy pairs (QGPs), with the redshift of the galaxies in the range
0.03<zg<0.18, selected from the SDSS. The HI 21-cm absorption was searched
towards the 9 sight lines with impact parameters ranging from 10 to 55 kpc
using GMRT. 21-cm absorption was detected only in one case i.e. towards the
Quasar (zq=2.625 SDSS J124157.54+633241.6)-galaxy (zg=0.143 SDSS
J124157.26+633237.6) pair with the impact parameter 11 kpc. The quasar sight
line in this case pierces through the stellar disk of a galaxy having near
solar metallicity (i.e (O/H)+12=8.7) and star formation rate uncorrected for
dust attenuation of 0.1 M_odot/yr. The quasar spectrum reddened by the
foreground galaxy is well fitted with the Milky Way extinction curve (with an
Av of 0.44) and the estimated HI column density is similar to the value
obtained from 21-cm absorption assuming spin temperature of 100K. Combining our
sample with the z<0.1 data available in the literature, we find the
detectability of 21-cm absorption with integrated optical depth greater than
0.1 km\s to be 50% for the impact parameter less than 20 kpc. Using the surface
brightness profiles and relationship between the optical size and extent of the
HI disk known for nearby galaxies, we conclude that in most of the cases of
21-cm absorption non-detection, the sight lines may not be passing through the
HI gas. We also find that in comparison to the absorption systems associated
with these QGPs, z<1 DLAs with 21-cm absorption detections have lower CaII
equivalent widths despite having higher 21-cm optical depths and smaller impact
parameters. This suggests that the current sample of DLAs may be a biased
population that avoids sight lines through dusty star-forming galaxies. A
systematic survey of QGPs is needed to confirm these findings and understand
the nature of 21-cm absorbers.
