The depth of the Lyman valley revisited

The severity of the intervening photoelectric absorption out to large redshift due to neutral hydrogen contained in quasar absorption line systems is reassessed in light of improved absorber statistics resulting from the HST Quasar Absorption Line Key Project. Compared to previous estimates, the updated statistics imply roughly a doubling of the anticipated fraction of the sky showing a given residual opacity at the wavelength of the He ii λ304 line. Because of the steepness of the quasar luminosity function, this slight lifting of the intergalactic cloud cover significantly affects the expected appearance of the high redshift quasar population at short far-UV wavelengths, resulting in somewhat more optimistic prospects for carrying out further studies of intergalactic He ii absorption beyond the four objects toward which such absorption has been detected so far.