2010MNRAS.408..849G

We present the results from our 21-cm absorption survey of a sample of five quasar-galaxy pairs (QGPs), with the redshift of the galaxies in the range 0.03 ≤ z_g ≤ 0.18, selected from the Sloan Digital Sky Survey (SDSS). The Hi 21-cm absorption was searched towards the nine sightlines with impact parameters ranging from ∼10 to ∼55 kpc using the Giant Metrewave Radio Telescope (GMRT). 21-cm absorption was detected only in one case, i.e. towards the quasar (z_q= 2.625 SDSS J124157.54+633241.6)-galaxy (z_g= 0.143 SDSS J124157.26+633237.6) pair with the impact parameter ∼11 kpc. The quasar sightline in this case pierces through the stellar disc 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_☉/yr. The quasar spectrum reddened by the foreground galaxy is well fitted with the Milky Way extinction curve (with an A_V of 0.44) and the estimated Hi column density is similar to the value obtained from 21-cm absorption assuming a spin temperature (T_S) of 100 K.

In the remaining cases, our GMRT spectra provide upper limit on N(Hi) in the range (10¹⁷-10¹⁸) xT_S/cm2. 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 per cent for the impact parameter less than 20 kpc. Using the surface brightness profiles and a well-established relationship between the optical size and extent of the Hi disc known for nearby galaxies, we conclude that in most of the cases of 21-cm absorption non-detection, the sightlines may not be passing through the Hi gas (1σ column density of a few times 10¹⁹/cm2). We also find that in comparison to the absorption systems associated with these QGPs, z < 1 damped Lyman-α absorbers (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 sightlines through dusty star-forming galaxies. A systematic survey of QGPs over a wider redshift range using a large sample is needed to confirm these findings and understand the nature of 21-cm absorbers.