Probing radio source environments via H I and OH absorption

Abstract

We present the results of HI and OH absorption measurements towards a sample of radio sources using the Arecibo 305-m telescope and the Giant Metrewave Radio Telescope (GMRT). In total, 27 radio sources were searched for associated 21 -cm H I absorption. One totally new H I absorption system was detected against the radio galaxy 3C258, while five previously known Hi absorption systems, and one galaxy detected in emission, were studied with improved frequency resolution and/or sensitivity. Our sample included 17 gigahertz peaked spectrum (GPS) and compact steep spectrum (CSS) objects, four of which exhibit H I absorption. This detection rate of ∼25 per cent compares with a value of ∼40 per cent by Vermeulen et al. for similar sources. We detected neither OH emission nor absorption towards any of the sources that were observed at Arecibo. We are, however, able to estimate a limit on the abundance ratio of N(H I)/N(OH) ≳ 4 × 106 for 3C258. We have combined our results with those from other available H I searches to compile a heterogeneous sample of 96 radio sources consisting of 27 GPS, 35 CSS, 13 compact flat spectrum (CFS) and 21 large (LRG) sources. The Hi absorption detection rate is highest (∼45 per cent) for the compact GPS sources and least for the LRG sources. We find H I column density to be anticorrelated with source size, as reported earlier by Pihlström et al., a trend which is consistent with the results of optical spectroscopy. The H I column density shows no significant dependence on either redshift or luminosity, which are themselves strongly correlated. These results suggest that the environments of radio sources on GPS/CSS scales are similar at different redshifts. Further, in accordance with the unification scheme, the GPS/CSS galaxies have an H I detection rate of ∼40 per cent which is higher than the detection rate (∼20 per cent) towards the GPS/CSS quasars. Also, the principal (strongest) absorption component detected towards GPS sources appears blueshifted in ∼65 per cent of the cases. This is in agreement with the growing evidence for jet-cloud interactions playing an important role in determining the ionization and kinematical properties of the ambient gas. © 2006 RAS.