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

TL;DR

This study compares HI 21-cm absorption profiles in redshifted galaxies, revealing that associated absorbers are generally wider and more complex than intervening ones, and demonstrates machine learning can predict absorber type with over 80% accuracy.

Contribution

It provides the first comprehensive analysis of profile shapes distinguishing associated and intervening absorbers and introduces machine learning methods for predicting absorber type without optical data.

Findings

Associated profiles are wider than intervening ones.

Additional low optical depth component arises from gas within the inner parsec.

Machine learning models achieve over 80% accuracy in classifying absorber type.

Abstract

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.