A survey for redshifted molecular and atomic absorption lines - II. Associated H i, OH and millimetre lines in the z ≳ 3 Parkes quarter-Jansky flat-spectrum sample

We present the results of a z 2.9 survey for H I 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 H I 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 10 23 W Hz -1 , 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 10 23 W Hz -1 sources where there is a near 50 per cent detection rate of 21-cm absorption.