H I 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 H I 21-cm absorption, using Giant Metrewave Radio Telescope, Very Large Array and Westerbork Radio Synthesis Telescope, in a sample of 55 z &lt; 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, probed by 45 sightlines, we have found seven H I 21-cm absorption detections, two of which are reported here for the first time. Combining our primary samplewith measurements having similar optical depth sensitivity (∫ τdv =0.3 kms-1) from the literature, we find a weak anti-correlation (rank correlation coefficient=-0.20 at 2.42σ level) between ∫ τdv and b, consistent with previous literature results. The covering factor of H I 21-cm absorbers (C21) is estimated to be 0.24-0.08+0.12 at b ≤ 15 kpc and 0.06-0.04+0.09 at b = 15-35 kpc. ∫ τdv and C21 show similar declining trend with radial distance along the galaxy's major axis and distances scaled with the effective H I radius. There is also tentative indication that most of the H I 21-cm absorbers could be co-planar with the extended H I discs. No significant dependence of ∫ τdv and C21 on galaxy luminosity, stellar mass, colour and star formation rate is found, though the H I 21-cm absorbing gas cross-section may be larger for the luminous galaxies. The higher detection rate (by a factor of ~4) of H I 21-cm absorption in z &lt; 1 damped Lyman-α systems compared to the quasar-galaxy pairs indicates towards small covering factor and patchy distribution of cold gas clouds around low-z galaxies.