2020ApJ...899..163S

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_ν∝ν^-α_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^(HI) and f_esc^(HeII) 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^(HI)/f_esc^(HeII). 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^(HeII)~0.6–0.8 but f_esc^(HI)~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^(HeII)> 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.