Redshifted H I and OH absorption in radio galaxies and quasars

From a survey for the redshifted H I 21-cm and OH 18-cm absorption in the hosts of a sample of radio galaxies and quasars, we detect H I 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 L_UV≤ 10²³ W Hz^-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 L_UV > 10²³ W Hz^-1. Our 21-cm detections in combination with those previously published give a total of eight (associated and intervening) H I-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.