The mid-infrared properties and gas content of active galaxies over
  large look-back times
  Upon an expansion of all of the searches for redshifted HI 21-cm absorption
(0.0021 < z 5.19), we update recent results regarding the detection of 21-cm in
the non-local Universe. Specifically, we confirm that photo-ionisation of the
gas is the mostly likely cause of the low detection rate at high redshift, in
addition to finding that at z < 0.1 there may also be a decrease in the
detection rate, which we suggest is due to the dilution of the absorption
strength by 21-cm emission. By assuming that associated and intervening
absorbers have similar cosmological mass densities, we find evidence that the
spin temperature of the gas evolves with redshift, consistent with heating by
ultra-violet photons. From the near--infrared (3.4, 4.6 and 12 micron) colours,
we see that radio galaxies become more quasar-like in their activity with
increasing redshift. We also find that the non-detection of 21-cm absorption at
high redshift is not likely to be due to the selection of gas-poor ellipticals,
in addition to a strong correlation between the ionising photon rate and the
[3.4] - [4.6] colour, indicating that the UV photons arise from AGN activity.
Like previous studies, we find a correlation between the detection of 21-cm
absorption and the [4.6] - [12] colour, which is a tracer of star-forming
activity. However, this only applies at the lowest redshifts (z < 0.1), the
range considered by the other studies.
