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

TL;DR

This study finds an anti-correlation between HI 21-cm absorption strength and impact parameter in external galaxies, suggesting absorption occurs more in high-inclination disks and towards compact radio sources, with no significant variation in spin temperature across disks.

Contribution

It provides new observational evidence linking HI 21-cm absorption strength to impact parameter and galaxy inclination, and explores biases related to background radio source size.

Findings

Anti-correlation between absorption strength and impact parameter.

No significant variation in spin temperature across galaxy disks.

Preference for detection in high-inclination disks and towards compact radio sources.

Abstract

By combining the data from surveys for HI 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, we find no evidence that the spin temperature of the gas (degenerate with the covering factor) varies significantly across the disk. This is consistent with the uniformity of spin temperature measured across the Galactic disk. Furthermore, comparison with the Galactic distribution suggests that intervening 21-cm absorption preferentially arises in disks 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 5 x 10^8 Hz, compared to 1 x 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.