Softening of the Metagalactic Ionizing Background from Internal He II Absorption in Quasars
arXiv (Cornell University)(2020)
摘要
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. We also examine H I and He II photoelectric heating from the reprocessed background, whose spectral index (alpha_b > alpha_s) depends on alpha_s, the H I column density slope beta, and the ratio R = f_esc(HI) / f_esc(HeII). We compare the model to Lya 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 Delta-alpha_b = 0.3-1.0, corresponding to eta = 60-200 for beta = 1.5-1.6, in agreement with HST/COS observations at z = 2.5-2.9. The observed range of ratios, eta < 200, constrains alpha_b < 3.4 and f_esc(HeII) > 0.4. Individual AGN with softer spectra, alpha_s > 1.7, and more internal He II absorption could produce a few absorbers with eta > 300 in proximity to AGN transverse to the sight line.
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关键词
metagalactic ionizing background,quasars,absorption,internal the ii
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