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. 
S. J. Curran and M. T. Whiting
astro-ph.CO