Ionisation of the atomic gas in redshifted radio sources
  We report the results of a survey for HI 21-cm absorption at z < 0.4 in a new
sample of radio sources with the Giant Metrewave Radio Telescope. Of the 11
sources for which there are good data, we obtain zero detections, where four
are expected upon accounting for the ionising photon rates and sensitivity.
Adding these to the previously published values, we confirm that the
non-detection of 21-cm absorption in active sources at high redshift is due to
photo-ionisation of the gas rather than excitation by 21-cm photons
(significant at 6.09 sigma and 2.90 sigma, respectively). We also confirm a
strong correlation between the absorption strength and the reddening of the
source, suggesting that dust plays a significant role in shielding the gas from
the ambient ultra-violet field. An anti-correlation between the 21-cm detection
rate and the radio turnover frequency is also found, which runs contrary to
what is expected on the basis that the higher the turnover frequency, the more
compact the source. It is, however, consistent with the hypothesis that the
turnover frequency is related to the electron density, supported by a
correlation between the turnover frequency and ionising photon rate.
