Localized H I 21-cm absorption towards a double-lobed z=0.24 radio galaxy

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 γ ≈ 1216 Å luminosities are below LUV∼ 1023 W Hz-1, 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 (AGNs) 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 another intervening galaxy, or cool HI gas in the intergalactic medium, covering only the south-west (SW) 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 disc centred between the lobes, intervening the SW lobe while being located behind the north-east (NE) lobe, by assuming that it covers the emission peak at ≈150 kpc from the nucleus, we estimate a dynamical mass of ≈3 × 1012 M⊙ for the disc.