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

2020/07/06 by J. Michael Shull, Shull, Michael, Charles Danforth +1
Physics and Astronomy · #Astrophysical Phenomena and Observations #Astrophysics and Cosmic Phenomena #Astrophysics of Galaxies (astro-ph.GA) #Cosmology and Nongalactic Astrophysics (astro-ph.CO) #FOS: Physical sciences #Galaxies: Formation, Evolution, Phenomena #Gamma-ray bursts and supernovae #Stellar, planetary, and galactic studies

paper · pdf · doi:10.48550/arxiv.2007.02948

openalex publication_date 2020/07/06 · openalex created_date 2022/07/18 · openalex updated_date 2026/07/28

Abstract

Quasars and active galactic nuclei (AGN) are significant contributors to the\nmetagalactic ionizing background at redshifts z < 3. Recent HST/COS composite\nspectra of AGN find a harder flux distribution in the Lyman continuum, F_nu ~\nnu^-alpha_s (alpha_s = 1.41+/-0.15) compared to previous studies. This index\nappears to be inconsistent with observed He II / H I absorption ratios (eta) in\nthe Lya forest. We explore effects of internal AGN absorption in the He II (4\nryd) continuum using an analytic source-function model of the ionizing\nbackground in which the emissivity (j_nu) arises from quasars, reprocessed by\nthe opacity (kappa_nu) of the intervening Lya forest and distinct AGN escape\nfractions f_esc(HI) and f_esc(HeII) at 1~ryd and 4~ryd, respectively. We also\nexamine H I and He II photoelectric heating from the reprocessed background,\nwhose spectral index (alpha_b > alpha_s) depends on alpha_s, the H I column\ndensity slope beta, and the ratio R = f_esc(HI) / f_esc(HeII). We compare the\nmodel to Lya absorption lines of He II and H I toward the quasar HE~2347-4342.\nInternal AGN absorption with f_esc(HeII) = 0.6-0.8 but f_esc(HI) = 1 would\nincrease the index by Delta-alpha_b = 0.3-1.0, corresponding to eta = 60-200\nfor beta = 1.5-1.6, in agreement with HST/COS observations at z = 2.5-2.9. The\nobserved range of ratios, eta < 200, constrains alpha_b < 3.4 and f_esc(HeII) >\n0.4. Individual AGN with softer spectra, alpha_s > 1.7, and more internal He II\nabsorption could produce a few absorbers with eta > 300 in proximity to AGN\ntransverse to the sight line.\n

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