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

Understand

Quasars and active galactic nuclei (AGN) are significant contributors to the metagalactic ionizing background at redshifts z < 3.

• Recent HST/COS composite spectra of AGN find a harder flux distribution in the Lyman continuum, F_nu ~ nu^{-alpha_s} (alpha_s = 1.41+/-0.15) compared to previous studies.
• This index appears to be inconsistent with observed He II / H I absorption ratios (eta) in the Lya forest.
• We explore 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_nu) arises from quasars, reprocessed by the opacity (kappa_nu) of the intervening Lya forest and distinct AGN escape fractions f_esc(HI) and f_esc(HeII) at 1~ryd and 4~ryd, respectively.