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 < 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 sample with measurements having similar optical depth sensitivity (∫τdv ≤ 0.3 km s^-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-cmabsorption in z<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. [ABSTRACT FROM AUTHOR]