Redshifted Hi and OH absorption in radio galaxies and quasars

Abstract

From a survey for the redshifted Hi 21-cm and OH 18-cm absorption in the hosts of a sample of radio galaxies and quasars, we detect Hi in three of the 10 and OH in none of the 14 sources for which useful data were obtained. As expected from our recent result, all of the 21-cm detections occur in sources with ultraviolet (UV) continuum luminosities of LUV≤ 1023WHz-1. At these 'moderate' luminosities, we also obtain four non-detections, although, as confirmed by the equipartition of detections between the type 1 and type 2 objects, this near-50 per cent detection rate cannot be attributed to unified schemes of active galactic nuclei (AGNs). All of our detections are at redshifts of z≲ 0.67, which, in conjunction with our faint source selection, biases against UV luminous objects. The importance of the UV luminosity (over AGN type) in the detection of the 21-cm absorption is further supported by the non-detections in the two high-redshift (z∼ 3.6-3.8) radio galaxies, which are both type 2 objects, while having LUV > 1023 W Hz-1. Our 21-cm detections in combination with those previously published give a total of eight (associated and intervening) Hi-absorbing sources searched and undetected in OH. Using the detected 21-cm line strengths to normalize the limits, we find that only two of these eight sources may have been searched sufficiently deeply in OH, even though these are marginal.