  Cool neutral gas provides the raw material for all star formation in the
Universe, and yet, from a survey of the hosts of high redshift radio galaxies
and 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 the
optical photometry, we find that all of our targets have ionising ultra-violet
continuum luminosities of logL > 23 W/Hz. We therefore attribute this deficit
to the traditional optical selection of targets biasing surveys towards the
most ultra-violet luminous objects, where the intense radiation excites the
neutral gas to the point where it cannot engage in star formation. However,
this hypothesis does not explain why there is a critical luminosity, rather
than a continuum where the detections gradually become fewer and fewer as the
harshness of the radiation increases. We show that by placing a quasar within a
galaxy of gas there is always a finite ultra-violet luminosity above which all
of the gas is ionised. This demonstrates that these galaxies are probably
devoid of star-forming material rather than this being at abundances below the
sensitivity limits of current radio telescopes.
