Redshifted HI and OH absorption in radio galaxies and quasars

TL;DR

This study investigates redshifted HI and OH absorption in radio galaxies and quasars, finding HI absorption mainly in sources with moderate UV luminosity and highlighting the influence of UV luminosity over AGN type on detection rates.

Contribution

It provides new observational data on HI and OH absorption in radio sources, emphasizing the role of UV luminosity in absorption detection, independent of AGN classification.

Findings

HI detected in 3 of 10 sources with useful data

All HI detections occur at UV luminosities below 10^23 W/Hz

No OH absorption detected in any of the sources

Abstract

From a survey for 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 ten and OH in none of the fourteen sources for which useful data were obtained. As expected from our recent result, all of the 21-cm detections occur in sources with ultra-violet continuum luminosities of L < 10^23 W/Hz. 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% detection rate cannot be attributed to unified schemes of active galactic nuclei (AGN). 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 ultra-violet luminosity (over AGN type) in the detection of 21-cm 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 > 10^23 W/Hz. 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 normalise the limits, we find that only two of these eight may have been searched sufficiently deeply in OH, although even these are marginal.