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 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 similar to 10 to similar 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 (zg = 0.143 SDSS J124157.26+633237.6) pair with the impact parameter similar to 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-circle dot yr(-1). 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 a spin temperature (TS) of 100 K. In the remaining cases, our GMRT spectra provide upper limit on N(H I) in the range (10171018) x TS cm(-2). 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(-1) 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 (1 sigma column density of a few times 1019 cm(-2)). We also find that in comparison to the absorption systems associated with these QGPs, z < 1 damped Lyman-alpha absorbers (DLAs) with 21-cm absorption detections have lower Ca II 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.