The He II Lyman-alpha opacity of the universe

Abstract

view Abstract Citations (84) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The He II Lyman-Alpha Opacity of the Universe Madau, Piero ; Meiksin, Avery Abstract We discuss the recent Hubble Space Telescope 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 Lyα line of He II. We assess two possible explanations for this flux reduction: line blanketing produced by He II in the Lyα 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~3, well after the H II region network has fully percolated at z~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 Lyα 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 redshift must be ionized locally. Publication: The Astrophysical Journal Pub Date: October 1994 DOI: 10.1086/187546 Bibcode: 1994ApJ...433L..53M Keywords: Absorption Spectra; Cosmology; Helium; Lyman Alpha Radiation; Opacity; Quasars; Ultraviolet Astronomy; Ultraviolet Spectra; Baryons; Faint Object Camera; Gunn Effect; Hubble Space Telescope; Hydrogen; Intergalactic Media; Photoionization; Astronomy; COSMOLOGY: DIFFUSE RADIATION; COSMOLOGY: OBSERVATIONS; GALAXIES: INTERGALACTIC MEDIUM; GALAXIES: QUASARS: ABSORPTION LINES full text sources ADS | data products SIMBAD (2) NED (1)