SIMBAD references

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 1.5keV implies a maximum accreted-mass density onto massive black holes of ρ_acc≲1.4x10⁴M_☉M/pc3 for z>5. Considering the contribution of lower-z AGNs, the value reduces to ρ_acc≲0.66x10⁴M_☉M/pc3. 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.