H i 21-cm absorption survey of quasar-galaxy pairs: distribution of cold gas around<i>z</i> &lt; 0.4 galaxies

Abstract

We present the results from our survey of H &lt;scp&gt;i&lt;/scp&gt; 21-cm absorption, using Giant Metrewave Radio Telescope, Very Large Array and Westerbork Radio Synthesis Telescope, in a sample of 55 &lt;it&gt;z&lt;/it&gt; &lt; 0.4 galaxies towards radio sources with impact parameters (&lt;it&gt;b&lt;/it&gt;) 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 &lt;scp&gt;i&lt;/scp&gt; 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 (∫τd&lt;it&gt;v&lt;/it&gt; ≤ 0.3 km s−1) from the literature, we find a weak anti-correlation (rank correlation coefficient = −0.20 at 2.42σ level) between ∫τd&lt;it&gt;v&lt;/it&gt; and &lt;it&gt;b&lt;/it&gt;, consistent with previous literature results. The covering factor of H &lt;scp&gt;i&lt;/scp&gt; 21-cm absorbers (&lt;it&gt;C&lt;/it&gt;&lt;inf&gt;21&lt;/inf&gt;) is estimated to be 0.24&lt;f&gt;$^{+0.12}_{-0.08}$&lt;/f&gt; at &lt;it&gt;b&lt;/it&gt; ≤ 15 kpc and 0.06&lt;f&gt;$^{+0.09}_{-0.04}$&lt;/f&gt; at &lt;it&gt;b&lt;/it&gt; = 15−35 kpc. ∫τd&lt;it&gt;v&lt;/it&gt; and &lt;it&gt;C&lt;/it&gt;&lt;inf&gt;21&lt;/inf&gt; show similar declining trend with radial distance along the galaxy's major axis and distances scaled with the effective H &lt;scp&gt;i&lt;/scp&gt; radius. There is also tentative indication that most of the H &lt;scp&gt;i&lt;/scp&gt; 21-cm absorbers could be co-planar with the extended H &lt;scp&gt;i&lt;/scp&gt; discs. No significant dependence of ∫τd&lt;it&gt;v&lt;/it&gt; and &lt;it&gt;C&lt;/it&gt;&lt;inf&gt;21&lt;/inf&gt; on galaxy luminosity, stellar mass, colour and star formation rate is found, though the H &lt;scp&gt;i&lt;/scp&gt; 21-cm absorbing gas cross-section may be larger for the luminous galaxies. The higher detection rate (by a factor of ∼4) of H &lt;scp&gt;i&lt;/scp&gt; 21-cm absorption in &lt;it&gt;z&lt;/it&gt; &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-&lt;it&gt;z&lt;/it&gt; galaxies.