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