Abstract
ABSTRACT In this work we investigate the properties of the sources that reionized the intergalactic medium (IGM) in the high-redshift Universe. Using a semi-analytical model aimed at reproducing galaxies and black holes in the first ∼1.5 Gyr of the Universe, we revisit the relative role of star formation and black hole accretion in producing ionizing photons that can escape into the IGM. Both star formation and black hole accretion are regulated by supernova feedback, resulting in black hole accretion being stunted in low-mass haloes. We explore a wide range of combinations for the escape fraction of ionizing photons (redshift-dependent, constant, and scaling with stellar mass) from both star formation ($\langle f_{\rm esc}^{\rm sf} \rangle$) and AGN ($f_{\rm esc}^{\rm bh}$) to find: (i) the ionizing budget is dominated by stellar radiation from low stellar mass ($M_*\lt 10^9 \, {\rm \rm M_\odot }$) galaxies at z > 6 with the AGN contribution (driven by $M_{bh}\gt 10^6 \, {\rm \rm M_\odot }$ black holes in $M_* \gtrsim 10^9\, {\rm \rm M_\odot }$ galaxies) dominating at lower redshifts; (ii) AGN only contribute $10-25{{\ \rm per\ cent}}$ to the cumulative ionizing emissivity by z = 4 for the models that match the observed reionization constraints; (iii) if the stellar mass dependence of $\langle f_{\rm esc}^{\rm sf} \rangle$ is shallower than $f_{\rm esc}^{\rm bh}$, at z < 7 a transition stellar mass exists above which AGN dominate the escaping ionizing photon production rate; (iv) the transition stellar mass decreases with decreasing redshift. While AGN dominate the escaping emissivity above the knee of the stellar mass function at z ∼ 6.8, they take-over at stellar masses that are a tenth of the knee mass by z = 4.
Highlights
The epoch of reionization (EoR) begins when the first stars start producing neutral hydrogen (H I) ionizing photons and carving out ionized regions in the intergalactic medium (IGM)
Using a semi-analytical model aimed at reproducing galaxies and black holes in the first ∼1.5 Gyr of the Universe, we revisit the relative role of star formation and black hole accretion in producing ionizing photons that can escape into the IGM
We have studied the contribution of AGN to hydrogen reionization
Summary
The epoch of (hydrogen) reionization (EoR) begins when the first stars start producing neutral hydrogen (H I) ionizing photons and carving out ionized regions in the intergalactic medium (IGM). A similar scenario, where more than 50 per cent of the ionizing photons are emitted by rare and bright sources, such as quasars, has been proposed by Chardin et al (2015), Chardin, Puchwein & Haehnelt (2017) as a possible explanation of the large fluctuations in the Ly α effective optical depth on scales of 50 h−1 cMpc measured at the end stages of reionization (4 < z < 6) by Becker et al (2015) These AGN-dominated or AGN-assisted models, are found to reionize helium (He II) too early (Puchwein et al 2019) and result in an IGM temperature evolution that is inconsistent with the observational constraints (Becker et al 2011).
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