A survey for the missing hydrogen in high redshift radio sources
  Unlike at lower redshift, where there is a 40% detection rate, surveys for 21-cm absorption arising within the hosts of z > 1 radio galaxies and quasars have been remarkably unsuccessful. Curran et al.(2008) suggest that this is due to the high redshift selection biasing towards the most optically bright objects (those most luminous in the ultra-violetin the rest-frame), where the gas is ionised by the active galactic nucleus. They therefore argue that there must be a population of fainter objects in which the hydrogen is not ionised and which exhibit a similar detection rate as at lower redshifts. In order to find this "missing" gas at high redshift, we have therefore undertaken a survey of z > 2 radio sources, selected by optical faintness. Despite having optical magnitudes which indicate that the targets have ultra-violet luminosities below the threshold where all of the gas is ionised, there were no detections in any of the eight sources for which useable data were obtained. Upon an analysis of the spectral energy distributions, ionising photon rates can only be determined for three of these, all of which suggest that the objects are above the highest luminosity of a current 21-cm detection. The possibility that the other five could be located at lower photon rates cannot be ruled out, although zero detections out of five is not statistically significant. Another possible cause of the non-detections is that our selection biases the sample towards sources which are very steep in the radio band, with a mean spectral index of = -1.0, cf. -0.3 for both the 21-cm detections and UV luminous non-detections. This adds the further possibility that the sources have very extended emission, which would have the effect of reducing the coverage by the putative absorbing gas, thus decreasing the sensitivity of the observation. 
S. J. Curran, M. T. Whiting, E. M. Sadler and C. Bignell
astro-ph.CO