Massive black holes in high-redshift Lyman Break Galaxies

Several evidences indicate that Lyman Break Galaxies (LBGs) in the Epoch of Reionization (redshift z > 6) might host massive black holes (MBHs). 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 an M-h approximate to 10(12) M-circle dot halo at z = 6 host an MBH with mass M-circle dot approximate to 2 x 10(8) M-circle dot. Depending on the fraction, f(seed), of early haloes planted with a direct collapse black hole seed (M-seed = 10(5) M-circle dot), the model suggests two possible scenarios: (i) if f(seed) = 1, MBHs in LBGs mostly grow by merging and must accrete at a low (lambda(E) similar or equal to 10(-3)) Eddington ratio not to exceed the experimental X-ray luminosity upper bound L-X* = 10(42)(.5)erg s(-1); (ii) if f(seed) = 0.05, accretion dominates (lambda(E) similar or equal to 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 greater than or similar to - 23, with BH emission playing a subdominant role. Scenario (i) poses extremely challenging, and possibly unphysical, requirements on DCBH formation. Scenario (ii) entails testable implications on the physical properties of LBGs involving the FIR luminosity, emission lines, and the presence of outflows.

Automated summary

Evidence suggests that Lyman Break Galaxies (LBGs) in the Epoch of Reionization with redshift z > 6 may host massive black holes (MBHs). A merger-tree model combined with constraints from surveys and known populations of high-z super-MBHs reveals two possible scenarios for the growth and emission of MBHs in LBGs, depending on the fraction of early haloes planted with direct collapse black hole seeds. These scenarios have implications on the UV luminosity function and physical properties of LBGs.