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From the first published z > 3 survey of 21-cm absorption within the hosts ofradio galaxies and quasars, we found an apparent dearth of cool neutral gas athigh redshift. From a detailed analysis of the photometry, each object is foundto have a 1216 A continuum luminosity in excess of L ~1e23 W/Hz, a criticalvalue above which 21-cm has never been detected at any redshift. At thesewavelengths, and below, hydrogen is excited above the ground state so that itcannot absorb in 21-cm. In order to apply the equation of photoionsationequilibrium, we demonstrate that this critical value also applies to theionising (< 912 A) radiation. We use this to show, for a variety of gas densitydistributions, that upon placing a quasar within a galaxy of gas there isalways an ultra-violet luminosity above which all of the gas in the galaxy isionised. While in this state the hydrogen cannot be detected nor cannot engagein star formation. Applying the mean ionising photon rate of all of the sourcessearched, we find, using canonical values for the gas density and recombinationrate coefficient, that the observed critical luminosity gives a scale-length (3kpc) similar that of the neutral hydrogen (HI) in the Milky Way, a large spiralgalaxy. Thus, this simple, yet physically motivated, model can explain thecritical luminosity, above which neutral gas is not detected. This indicatesthat these galaxies are probably devoid of star-forming material, rather thenon-detection of 21-cm being due to the sensitivity limits of current radiotelescopes.

Complete ionisation of the neutral gas: why there are so few detections of 21-cm hydrogen in high redshift radio galaxies and quasars