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By combining the data from surveys for HI 21-cm absorption at various impactparameters in near-by galaxies, we report an anti-correlation between the 21-cmabsorption strength (velocity integrated optical depth) and the impactparameter. Also, by combining the 21-cm absorption strength with that of theemission, giving the neutral hydrogen column density, we find no evidence thatthe spin temperature of the gas (degenerate with the covering factor) variessignificantly across the disk. This is consistent with the uniformity of spintemperature measured across the Galactic disk. Furthermore, comparison with theGalactic distribution suggests that intervening 21-cm absorption preferentiallyarises in disks of high inclinations (near face-on). We also investigate thehypothesis that 21-cm absorption is favourably detected towards compact radiosources. Although there is insufficient data to determine whether there is ahigher detection rate towards quasar, rather than radio galaxy, sight-lines,the 21-cm detections intervene objects with a mean turnover frequency of 5 x10^8 Hz, compared to 1 x 10^8 Hz for the non-detections. Since the turnoverfrequency is anti-correlated with radio source size, this does indicate apreferential bias for detection towards compact background radio sources.

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