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Cool neutral gas provides the raw material for all star formation in theUniverse, and yet, from a survey of the hosts of high redshift radio galaxiesand quasars, we find a complete dearth of atomic (HI 21-cm) and molecular (OH,CO, HCO+ & HCN) absorption at redshifts z > 3. Upon a thorough analysis of theoptical photometry, we find that all of our targets have ionising ultra-violetcontinuum luminosities of logL > 23 W/Hz. We therefore attribute this deficitto the traditional optical selection of targets biasing surveys towards themost ultra-violet luminous objects, where the intense radiation excites theneutral gas to the point where it cannot engage in star formation. However,this hypothesis does not explain why there is a critical luminosity, ratherthan a continuum where the detections gradually become fewer and fewer as theharshness of the radiation increases. We show that by placing a quasar within agalaxy of gas there is always a finite ultra-violet luminosity above which allof the gas is ionised. This demonstrates that these galaxies are probablydevoid of star-forming material rather than this being at abundances below thesensitivity limits of current radio telescopes.

Complete Ionisation of the Neutral Gas in High Redshift Radio Galaxies and Quasars