Softening of the Metagalactic Ionizing Background from Internal He ii Absorption in Quasars

• Published in: The Astrophysical journal 2020-08, Vol.899 (2), p.163
• Main Authors: Shull, J. Michael, Danforth, Charles W.
• Format: Article
• Language: English
• Subjects: Absorption; Active galactic nuclei; Astrophysics; Emissivity; Hubble Space Telescope; Intergalactic medium; Opacity; Photoelectricity; Quasars; Radio quiet quasars; Space telescopes; Spectra
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aba3c9

Quasars and other active galactic nuclei (AGN) are significant contributors to the metagalactic ionizing background at redshifts z < 3. Recent Hubble Space Telescope (HST)/Cosmic Origins Spectrograph (COS) composite spectra of AGN find a harder flux distribution in the Lyman continuum, ( s = 1.41 0.15), compared to previous studies. This index appears to be inconsistent with observed He ii/H i absorption ratios ( ) in the Ly forest. We explore the effects of internal AGN absorption in the He ii (4 ryd) continuum using an analytic source-function model of the ionizing background in which the emissivity (j ) arises from quasars, reprocessed by the opacity (κ ) of the intervening Ly forest and distinct AGN escape fractions and at 1 ryd and 4 ryd, respectively. We also examine H i and He ii photoelectric heating from the reprocessed background, whose spectral index ( b > s) depends on s, the H i column density slope β, and the ratio . We compare the model to Ly absorption lines of He ii and H i toward the quasar HE 2347−4342. Internal AGN absorption with but would increase the index by Δ b 0.3-1.0, corresponding to = 60-200 for β 1.5-1.6, in agreement with HST/COS observations at z 2.5-2.9. The observed range of ratios, < 200, constrains b < 3.4 and . Individual AGN with softer spectra, s > 1.7, and more internal He ii absorption could produce a few absorbers with > 300, in proximity to AGN transverse to the sight line.