The Contribution of AGN Accretion Disks to Hydrogen Reionization

Abstract We examine the contribution of high-redshift ( z > 6) active galactic nuclei (AGNs) to cosmic hydrogen reionization, by tracing the growth and ionizing output of the first generation of supermassive black holes (SMBHs). Our calculations are anchored to the observed population of z ≃ 6 qu...

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Bibliographic Details
Published in:The Astrophysical journal 2022-04, Vol.929 (1), p.21
Main Authors: Zeltyn, Grisha, Trakhtenbrot, Benny
Format: Article
Language:English
Subjects:
Accretion
Accretion disks
Active galactic nuclei
Astrophysics
Black holes
Early universe
Evolution
Galaxies
Hydrogen
Ionization
Ionizing radiation
Luminosity
Quasars
Red shift
Reionization
Space density
Star formation
Supermassive black holes
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Summary:Abstract We examine the contribution of high-redshift ( z > 6) active galactic nuclei (AGNs) to cosmic hydrogen reionization, by tracing the growth and ionizing output of the first generation of supermassive black holes (SMBHs). Our calculations are anchored to the observed population of z ≃ 6 quasars, and trace back the evolving spectral energy distributions (SEDs) of the accretion flows that power these early AGNs and consider a variety of growth histories, including super-Eddington accretion. Compared to a fixed-shape SED, the evolving thin disks produce ionizing radiation that is higher by up to ∼80%. Across a variety of SMBH growth scenarios, the contribution of AGNs to reionization is limited to late epochs ( z < 7), and remains subdominant compared to star-forming galaxies. This conclusion holds irrespective of the (still unknown) space density of low-luminosity z = 6 AGNs, and for growth scenarios that allow super-Eddington accretion. The contribution of AGNs to reionization can extend to earlier epochs ( z ≳ 8) in scenarios with relatively slow SMBH mass growth, i.e., for low accretion rates and/or high spins. We finally demonstrate that our framework can reproduce the observed quasar proximity-zone sizes, and that compact proximity zones around z = 6 quasars can be explained by the late onset of super-Eddington accretion.
ISSN:0004-637X
1538-4357
DOI:10.3847/1538-4357/ac4783