Localised HI 21-cm absorption towards a double-lobed z=0.24 radio galaxy

TL;DR

This study conducted a mini-survey for HI 21-cm absorption in low-redshift radio galaxies, finding limited detections and suggesting that absorption is not solely determined by AGN unified schemes, with some evidence of localized absorption in a specific galaxy.

Contribution

First mini-survey targeting low-luminosity radio galaxies for HI 21-cm absorption, revealing localized absorption and challenging existing AGN unification models.

Findings

Two out of three sources showed no absorption.

Detected absorption towards one radio lobe in PKS 1649-062.

Estimated a dynamical mass of ~3 x 10^12 solar masses for a potential disk.

Abstract

We present the results of a mini-survey for associated HI 21-cm absorption at z < 0.42 with the Giant Metrewave Radio Telescope. Our targets are radio galaxies, selected on the basis that the 1216 Angstrom luminosities are below 10^23 W/Hz, above which there has never been a detection of 21-cm absorption. Of the three sources for which we obtained good data, two are unclassified active galactic nuclei (AGN) and one is type-2. Being a non-detection, the type-2 object is consistent with our previous result that 21-cm absorption in radio sources is not dictated by unified schemes of AGN. In the case of the detection, the absorption only occurs towards one of the two resolved radio lobes in PKS 1649-062. If the absorption is due to an another intervening galaxy, or cool HI gas in the intergalactic medium, covering only the south-west lobe, then, being at the same redshift, this is likely to be gravitationally bound to the optical object identified as PKS 1649-062. If the absorption is due to an inclined disk centred between the lobes, intervening the SW lobe while being located behind the NE lobe, by assuming that it covers the emission peak at 150 kpc from the nucleus, we estimate a dynamical mass of ~3 x 10^12 solar masses for the disk.