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The star-forming reservoir in the distant Universe can be detected through HI21-cm absorption arising from either cool gas associated with a radio source orfrom within a galaxy intervening the sight-line to the continuum source. Inorder to test whether the nature of the absorber can be predicted from theprofile shape, we have compiled and analysed all of the known redshifted (z >0.1) HI 21-cm absorption profiles. Although between individual spectra there istoo much variation to assign a typical spectral profile, we confirm thatassociated absorption profiles are on average, wider than their interveningcounterparts. It is widely hypothesised that this is due to high velocitynuclear gas feeding the central engine, absent in the more quiescentintervening absorbers. Modelling the column density distribution of the meanassociated and intervening spectra, we confirm that the additional low opticaldepth, wide dispersion component, typical of associated absorbers, arises fromgas within the inner parsec. With regard to the potential of predicting theabsorber type in the absence of optical spectroscopy, we have implementedmachine learning techniques to the 55 associated and 43 intervening spectra,with each of the tested models giving a >80% accuracy in the prediction of theabsorber type. Given the impracticability of follow-up optical spectroscopy ofthe large number of 21-cm detections expected from the next generation of largeradio telescopes, this could provide a powerful new technique with which todetermine the nature of the absorbing galaxy.

A comparative study of intervening and associated HI 21-cm absorption profiles in redshifted galaxies