Complete ionisation of the neutral gas: why there are so few detections
  of 21-cm hydrogen in high redshift radio galaxies and quasars
  From the first published z > 3 survey of 21-cm absorption within the hosts of
radio galaxies and quasars, we found an apparent dearth of cool neutral gas at
high redshift. From a detailed analysis of the photometry, each object is found
to have a 1216 A continuum luminosity in excess of L ~1e23 W/Hz, a critical
value above which 21-cm has never been detected at any redshift. At these
wavelengths, and below, hydrogen is excited above the ground state so that it
cannot absorb in 21-cm. In order to apply the equation of photoionsation
equilibrium, we demonstrate that this critical value also applies to the
ionising (< 912 A) radiation. We use this to show, for a variety of gas density
distributions, that upon placing a quasar within a galaxy of gas there is
always an ultra-violet luminosity above which all of the gas in the galaxy is
ionised. While in this state the hydrogen cannot be detected nor cannot engage
in star formation. Applying the mean ionising photon rate of all of the sources
searched, we find, using canonical values for the gas density and recombination
rate coefficient, that the observed critical luminosity gives a scale-length (3
kpc) similar that of the neutral hydrogen (HI) in the Milky Way, a large spiral
galaxy. Thus, this simple, yet physically motivated, model can explain the
critical luminosity, above which neutral gas is not detected. This indicates
that these galaxies are probably devoid of star-forming material, rather the
non-detection of 21-cm being due to the sensitivity limits of current radio
telescopes.
