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

Abstract

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, ${F}_{\nu }\propto {\nu }^{-{\alpha }_{s}}$ (α_s = 1.41 ± 0.15), compared to previous studies. This index appears to be inconsistent with observed He II/H I absorption ratios (η) in the Lyα 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_ν) arises from quasars, reprocessed by the opacity (κ_ν) of the intervening Lyα forest and distinct AGN escape fractions ${f}_{\mathrm{esc}}^{({\rm{H}}\,{\rm{\small{I}}})}$ and ${f}_{\mathrm{esc}}^{(\mathrm{He}\,{\rm{\small{II}}})}$ at 1 ryd and 4 ryd, respectively. We also examine H I and He II photoelectric heating from the reprocessed background, whose spectral index (α_b > α_s) depends on α_s, the H I column density slope β, and the ratio $R={f}_{\mathrm{esc}}^{({\rm{H}}\,{\rm{\small{I}}})}/{f}_{\mathrm{esc}}^{(\mathrm{He}\,{\rm{\small{II}}})}$. We compare the model to Lyα absorption lines of He II and H I toward the quasar HE 2347-4342. Internal AGN absorption with ${f}_{\mathrm{esc}}^{(\mathrm{He}\,{\rm{\small{II}}})}\approx 0.6\mbox{--}0.8$ but ${f}_{\mathrm{esc}}^{({\rm{H}}\,{\rm{\small{I}}})}\approx 1$ would increase the index by Δα_b ≈ 0.3-1.0, corresponding to η = 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, η < 200, constrains α_b < 3.4 and ${f}_{\mathrm{esc}}^{(\mathrm{He}\,{\rm{\small{II}}})}\gt 0.4$. Individual AGN with softer spectra, α_s > 1.7, and more internal He II absorption could produce a few absorbers with η > 300, in proximity to AGN transverse to the sight line.