HI 21-cm absorption and unified schemes of active galactic nuclei
  In this paper we further explore the implications that this has for the
currently popular consensus that it is the orientation of the circumnuclear
obscuring torus which determines whether absorption is present along our
sight-line. The fact that at logL > 23 W/Hz, both type-1 and type-2 objects
exhibit a 50% probability of detection, suggests that this is not the case and
that the bias against detection of HI absorption in type-1 objects is due
purely to the inclusion of the logL > 23 W/Hz sources. Similarly, the
ultra-violet luminosities can also explain why the presence of 21-cm absorption
shows a preference for radio galaxies over quasars and the higher detection
rate in compact sources, such as CSS or GPS sources, may also be biased by the
inclusion of high-luminosity sources. Being comprised of all 21-cm searched
sources at z>0.1, this is a necessarily heterogeneous sample, the constituents
of which have been observed by various instruments. By this same token,
however, the dependence on the UV luminosity may be an all encompassing effect,
superseding the unified schemes model, although there is the possibility that
the exclusive 21-cm non-detections at high UV luminosities could be caused by a
bias towards gas-poor ellipticals. Additionally, the high UV fluxes could be
sufficiently exciting/ionising the HI above 21-cm detection thresholds,
although the extent to which this is related to the neutral gas deficit in
ellipticals is currently unclear.
