THE HE-II LYMAN-ALPHA OPACITY OF THE UNIVERSE

We discuss the recent Hubble Space Telescope FOC observations by Jakob sen et al. of a sharp drop in the spectrum of the high redshift (z = 3 .29) quasar Q0302-003 at the rest-frame wavelength of 304 angstrom, th e Lyalpha line of He II. We assess two possible explanations for this flux reduction: line blanketing produced by He II in the Lyalpha fores t, and the He II Gunn-Peterson effect from a diffuse intergalactic med ium (IGM). The observed He II trough can be produced entirely by line blanketing only if the absorbing clouds are velocity broadened and/or their distribution extends to very low column densities. A significant contribution from absorption in a diffuse IGM is likely. We show that a steep ionizing UV background is required to explain the reported He II absorption and the simultaneous absence of H I. Such a soft metaga lactic flux could arise from star-forming galaxies, decaying dark matt er, or QSOs, after including the effect of the continuum opacity from intervening absorption systems on the integrated spectrum. We describe an alternative interpretation, that the He II reionization of the uni verse is not completed until z approximately 3, well after the H II re gion network has fully percolated at z approximately 5, and find that it too requires a steep ionizing spectrum for the individual sources. Photoionization from QSOs detected in optical surveys may satisfy the H I and He II limits if the Lyalpha forest contains a significant frac tion of the baryons in the universe. If the ionizing background remain s soft shortward of the H II edge, metal-line absorbers at high redshi ft must be ionized locally.