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

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 $f_{esc}^{\left(HI\right)}$ and $f_{esc}^{\left(HeII\right)}$ 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 $R=f_{esc}^{\left(HI\right)}/f_{esc}^{\left(HeII\right)}$. We compare the model to Lyα absorption lines of He ii and H i toward the quasar HE 2347−4342. Internal AGN absorption with $f_{esc}^{\left(HeII\right)}\approx 0.6\text{--}0.8$ but $f_{esc}^{\left(HI\right)}\approx 1$ 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 $f_{esc}^{\left(HeII\right)}>0.4$. 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.