Atomic and Molecular Absorption in Redshifted Radio Sources
  We report on a survey for associated HI 21-cm and OH 18-cm absorption with
the Giant Metrewave Radio Telescope at redshifts z = 0.2-0.4. Although the low
redshift selection ensures that our targets are below the critical ultra-violet
luminosity, which is hypothesised to ionise all of the neutral gas in the host
galaxy, we do not obtain any detections in the six sources searched. Analysing
these in context of the previous surveys, in addition to the anti-correlation
with the ultra-violet luminosity (ionising photon rate), we find a correlation
between the strength of the absorption and the blue -- near-infrared colour, as
well as the radio-band turnover frequency. We believe that these are due to the
photo-ionisation of the neutral gas, an obscured sight-line being more
conducive to the presence of cold gas and the compact radio emission being
better intercepted by the absorbing gas, maximising the flux coverage,
respectively. Regarding the photo-ionisation, the compilation of the previous
surveys increases the significance of the critical ionising photon rate, above
which all of the gas in the host galaxy is hypothesised to be ionised, to >5
sigma. This reaffirms that this is an ubiquitous effect, which has profound
implications for the detection of neutral gas in these objects with the Square
Kilometre Array.
