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

Abstract

The star-forming reservoir in the distant Universe can be detected through H I 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) H I 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.

Hi absorption in nearby compact radio galaxies

“…We see shallow, broad components occur more often in early-type galaxies, as modelled in Curran et al (2016b). Two of our detections against such early-type galaxies lack any narrow and deep component [here and Allison et al (2013)], an attribute not seen in e.g.…”

FLASH early science – discovery of an intervening H i 21-cm absorber from an ASKAP survey of the GAMA 23 field

“…This is large compared with the distribution of line widths for intervening 21-cm absorbers in the literature (see e.g. Curran et al 2016b), and so unlikely to affect our detecting rate for these systems. However, this could be a factor in detecting intrinsic 21-cm absorbers associated with AGNs, which typically have broader line profiles (see subsection 3.4).…”

“…Since the sensitivity does depend on the choice of velocity width, we adopt a value equal to the mean for all intervening 21-cm absorbers detected in the literature, ∆v FWHM = 30 km s −1 (see e.g. Curran et al 2016b;Allison et al 2016b). In this case, we recover all lines that have a peak S/N greater than 5.5, and that is the value we adopt in the following analysis.…”

The potential of tracing the star formation history with H I 21-cm in intervening absorption systems

“…2). 4 Since there is no evolution in the FWHM of the intervening absorbers detected in 21-cm (Curran et al 2016a), we do not consider any redshift dependence.…”