2008MNRAS.391..765C

We present the results of a z ≥ 2.9 survey for Hi 21-cm and molecular absorption in the hosts of radio quasars using the Giant Metrewave Radio Telescope and the Tidbinbilla 70-m telescope. Although the atomic gas has been searched to limits capable of detecting most known absorption systems, no Hi was detected in any of the 10 sources. Previously published searches, which are overwhelmingly at redshifts of z ≲ 1, exhibit a 42 per cent detection rate (31 out of 73 sources), whereas the inclusion of our survey yields a 17 per cent detection rate (two out of 12 sources) at z > 2.5. We therefore believe that our high-redshift selection is responsible for our exclusive non-detections, and find that at ultraviolet (UV) luminosities of L_UV≳ 10²³W/Hz, 21-cm absorption has never been detected. We also find this to not only apply to our targets, but also those at low redshift exhibiting similar luminosities, giving zero detections out of a total of 16 sources over z = 0.24 to 3.8. This is in contrast to the L_UV≲ 10²³W/Hz sources where there is a near 50 per cent detection rate of 21-cm absorption.

The mix of 21-cm detections and non-detections is currently attributed to orientation effects, where according to unified schemes of active galactic nuclei, 21-cm absorption is more likely to occur in sources designated as radio galaxies (type 2 objects, where the nucleus is viewed through dense obscuring circumnuclear gas) than in quasars (type 1 objects, where we have a direct view to the nucleus). However, due to the exclusively high UV luminosities of our targets it is not clear whether orientation effects alone can wholly account for the distribution, although there exists the possibility that the large luminosities are indicative of a changing demographic of galaxy types. We also find that below luminosities of L_UV∼ 10²³W/Hz, both type 1 and type 2 objects have a 50 per cent likelihood of exhibiting 21-cm absorption.

Finally, we do not detect molecular gas in any of the sources. The lack of Hi absorption, combined with the results from Paper I, suggests these sources are not conducive to high molecular abundances.