Limits on the high redshift growth of massive black holes

Abstract

We place firm upper limits on the global accretion history of massive black holes at z > 5 from the recently measured unresolved fraction of the cosmic X-ray background. The maximum allowed unresolved intensity observed at 1.5 keV implies a maximum accreted-mass density onto massive black holes of ρ acc < 1.4 × 10 4 M Mpc -3 for z > 5. Considering the contribution of lower-z AGNs, the value reduces to ρ acc < 0.66 × 10 4 MMpc -3. The tension between the need for the efficient and rapid accretion required by the observation of massive black holes already in place at z > 7 and the strict upper limit on the accreted mass derived from the X-ray background may indicate that black holes are rare in high redshift galaxies or that accretion is only efficient for the black holes hosted by rare galaxies. © 2012 ESO.