  The star-forming reservoir in the distant Universe can be detected through HI
21-cm absorption arising from either cool gas associated with a radio source or
from within a galaxy intervening the sight-line to the continuum source. In
order to test whether the nature of the absorber can be predicted from the
profile shape, we have compiled and analysed all of the known redshifted (z >
0.1) HI 21-cm absorption profiles. Although between individual spectra there is
too much variation to assign a typical spectral profile, we confirm that
associated absorption profiles are on average, wider than their intervening
counterparts. It is widely hypothesised that this is due to high velocity
nuclear gas feeding the central engine, absent in the more quiescent
intervening absorbers. Modelling the column density distribution of the mean
associated and intervening spectra, we confirm that the additional low optical
depth, wide dispersion component, typical of associated absorbers, arises from
gas within the inner parsec. With regard to the potential of predicting the
absorber type in the absence of optical spectroscopy, we have implemented
machine learning techniques to the 55 associated and 43 intervening spectra,
with each of the tested models giving a >80% accuracy in the prediction of the
absorber type. Given the impracticability of follow-up optical spectroscopy of
the large number of 21-cm detections expected from the next generation of large
radio telescopes, this could provide a powerful new technique with which to
determine the nature of the absorbing galaxy.
