  We investigate the practice of assigning high spin temperatures to damped
Lyman-alpha absorption systems (DLAs) not detected in HI 21-cm absorption. In
particular, Kanekar & Chengaulr (2003) have attributed the mix of 21-cm
detections and non-detections in low redshift (z<2.04) DLAs to a mix of spin
temperatures, while the non-detections at high redshift were attributed to high
spin temperatures. Below z=0.9, where some of the DLA host galaxy morphologies
are known, we find that 21-cm absorption is normally detected towards large
radio sources when the absorber is known to be associated with a large
intermediate (spiral) galaxy. Furthermore, at these redshifts, only one of the
six 21-cm non-detections has an optical identification and these DLAs tend to
lie along the sight-lines to the largest background radio continuum sources.
For these and many of the high redshift DLAs occulting large radio continua, we
therefore expect covering factors of less than the assumed/estimated value of
unity. This would have the effect of introducing a range of spin temperatures
considerably narrower than the current range of >9000 K, while still supporting
the hypothesis that the high redshift DLA sample comprises a larger proportion
of compact galaxies than the low redshift sample.
