Massive black holes in high-redshift Lyman Break Galaxies

Abstract

Several evidences indicate that Lyman Break Galaxies (LBG) in the Epoch of Reionization (redshift z > 6) might host massive black holes (MBH). We address this question by using a merger-tree model combined with tight constraints from the 7 Ms Chandra survey, and the known high-z super-MBH population. We find that a typical LBG with M UV = −22 residing in a M h ≈ 10 12 M halo at z = 6 host a MBH with mass M • ≈ 2 × 10 8 M . Depending on the fraction, f seed , of early halos planted with a direct collapse black hole seed (M seed = 10 5 M ), the model suggests two possible scenarios: (a) if f seed = 1, MBH in LBGs mostly grow by merging, and must accrete at a low (λ E 10 −3 ) Eddington ratio not to exceed the experimental X-ray luminosity upper bound L * X = 10 42.5 erg s −1 ; (b) if f seed = 0.05 accretion dominates (λ E 0.22), and MBH emission in LBGs must be heavily obscured. In both scenarios the UV luminosity function is largely dominated by stellar emission up to very bright mag, M UV > ∼ − 23, with BH emission playing a subdominant role. Scenario (a) poses extremely challenging, and possibly unphysical, requirements on DCBH formation. Scenario (b) entails testable implications on the physical properties of LBGs involving the FIR luminosity, emission lines, and presence of outflows.

Semi-analytic forecasts for JWST -- V. AGN luminosity functions and helium reionization at z = 2-7

“…On the other hand, semi-analytic and empirical approaches provide flexible, physically motivated alternatives for efficiently modelling large ensembles of objects (e.g. Fanidakis et al 2013;Menci et al 2014;Ricarte & Natarajan 2018a;Dayal et al 2020;Fontanot et al 2020;Orofino et al 2021). Combining the high computational efficiency and the modularized structure of these models, controlled experiments can also be conducted to explore the parameter space and alternative prescriptions for specific physical processes.…”