Where is the cold neutral gas in the hosts of high redshift AGN? .

Previous surveys for HI 21-cm absorption in z > 0.1 radio galaxies and quasars yield a ≈40% detection rate, which is attributed to unified schemes of active galactic nuclei (AGN). In this paradigm absorption is only witnessed in (close to) type-2 objects, where the central obscuration is viewed (nearly) edge-on and thus absorbs the rest frame 1420 MHz emission along our sight-line. However, we find this mix of detections and non-detections to only apply at low redshift (z < 1): From a sensitive survey of eight z ≳3 radio sources we find no 21-cm absorption, indicating a low abundance of cold neutral gas in (the sight-lines searched in) these objects. Analysing the spectral energy distributions of these sources, we find that our high redshift selection introduces a bias where our sample consists exclusively of quasars with ultra-violet luminosities in excess of L_UV∼10²³ W Hz^-1. This may suggest that we have selected a class of particularly UV bright type-1 objects. Whatever the cause, it must also be invoked to explain the non-detections in an equal number of z < 0.7 sources, where we find, for the first time, the same exclusive non-detections at L_UV ≳ 10²³ W Hz^-1. These objects also turn out to be quasars and, from these exclusive high UV luminosity-21-cm non-detections, it is apparent that orientation effects alone cannot account for the mix of 21-cm detections and non-detections at any redshift.