Massive black holes in high-redshift Lyman Break Galaxies

TL;DR

This study models the presence and growth of massive black holes in high-redshift Lyman Break Galaxies, revealing two possible growth scenarios with distinct observational signatures and implications for early black hole formation.

Contribution

It introduces a merger-tree model combined with observational constraints to explore black hole growth in early galaxies, highlighting two distinct evolutionary scenarios.

Findings

MBHs in LBGs can reach ~2×10^8 M_sun at z=6.

Two growth scenarios depend on seed fraction and accretion rates.

Observable differences include FIR luminosity and emission line properties.

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 $M_{\rm UV}=-22$ residing in a $M_h\approx 10^{12} M_\odot$ halo at $z=6$ host a MBH with mass $M_\bullet \approx 2\times 10^8 M_\odot$. Depending on the fraction, $f_{\rm seed}$, of early halos planted with a direct collapse black hole seed ($M_{\rm seed}=10^5 M_\odot$), the model suggests two possible scenarios: (a) if $f_{\rm seed}=1$, MBH in LBGs mostly grow by merging, and must accrete at a low ($\lambda_E\simeq 10^{-3}$) Eddington ratio not to exceed the experimental X-ray luminosity upper bound $L_X^* = 10^{42.5} {\rm erg\, s}^{-1}$; (b) if $f_{\rm seed}=0.05$ accretion dominates ($\lambda_E\simeq 0.22$), 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, $M_{\rm UV} > -23$, 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.