A correlation between the H i 21-cm absorption strength and impact parameter in external galaxies

By combining the data from surveys for H i 21-cm absorption at various impact parameters in near-by galaxies, we report an anti-correlation between the 21-cm absorption strength (velocity integrated optical depth) and the impact parameter. Also, by combining the 21-cm absorption strength with that of the emission, giving the neutral hydrogen column density, $N_{{\rm H\,{\small I}}}$ , we find no evidence that the spin temperature of the gas (degenerate with the covering factor) varies significantly across the disc. This is consistent with the uniformity of spin temperature measured across the Galactic disc. Furthermore, comparison with the Galactic $N_{{\rm H\,{\small I}}}$ distribution suggests that intervening 21-cm absorption preferentially arises in discs of high inclinations (near face-on). We also investigate the hypothesis that 21-cm absorption is favourably detected towards compact radio sources. Although there is insufficient data to determine whether there is a higher detection rate towards quasar, rather than radio galaxy, sight-lines, the 21-cm detections intervene objects with a mean turnover frequency of $\langle \nu _{_{\rm TO}}\rangle \approx 5\times 10^{8}$ Hz, compared to $\langle \nu _{_{\rm TO}}\rangle \approx 1\times 10^{8}$ Hz for the non-detections. Since the turnover frequency is anti-correlated with radio source size, this does indicate a preferential bias for detection towards compact background radio sources.