Massive black holes in high-redshift Lyman Break Galaxies

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 7 Ms Chandra survey, and known high-$z$ super-MBH population. find typical LBG $M_{\rm UV}=-22$ residing $M_h\approx 10^{12} M_\odot$ halo at $z=6$ MBH mass $M_\bullet \approx 2\times 10^8 M_\odot$. Depending on fraction, $f_{\rm seed}$, early halos planted direct collapse hole seed ($M_{\rm seed}=10^5 M_\odot$), suggests two possible scenarios: (a) if seed}=1$, LBGs mostly grow merging, must accrete low ($\lambda_E\simeq 10^{-3}$) Eddington ratio not to exceed experimental X-ray luminosity upper bound $L_X^* = 10^{42.5} {\rm erg\, s}^{-1}$; (b) seed}=0.05$ accretion dominates 0.22$), emission be heavily obscured. In both scenarios UV function is largely dominated stellar up very bright mag, UV} > -23$, BH playing subdominant role. Scenario poses extremely challenging, possibly unphysical, requirements DCBH formation. entails testable implications physical properties involving FIR luminosity, lines, presence outflows.