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.
