The He II Lyman-alpha opacity of the universe

Abstract

We discuss the recent Hubble Space Telescope Faint Object Camera (FOC) observations by Jakobsen 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 A, the Lyman alpha line of He II. We assess two possible explanations for this flux reduction: line blanketing produced by He II in the Lyman alpha forest, and the He II Gunn-Peterson effect from a diffuse intergalactic medium (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 metagalactic flux could arise from star-forming galaxies, decaying dark matter, 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 universe is not completed until z approximately 3, well after the H II region 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 Lyman alpha forest contains a significant fraction of the baryons in the universe. If the ionizing background remains soft shortward of the H II edge, metal-line absorbers at high redhsift must be ionized locally.