On the He II Gunn–Peterson effect and the He II forest

The flux of a quasar below a rest-frame wavelength of 304 Å (the Ly α line of He II) should be reduced owing to the He II Gunn–Peterson effect and the He II absorption lines of the Ly α forest. This region of the spectrum may soon be observed for highredshift quasars with the HST, HUT and other satellites. The predicted decrement depends mainly on the spectrum of the ionizing background at high redshift. If this background is produced by quasars with a power-law spectrum joining their observed UV and X-ray luminosities, the He II decrement should be about four times larger than the decrement of the H I Ly α forest, being a factor of ~3 at z = 3, and should be easily seen on any quasar detected down to the He II Ly α wavelength. The observation of a significantly lower decrement would imply that the sources producing the ionizing background have a harder spectrum than in the simple case mentioned above.