Limits on the high redshift growth of massive black holes

TL;DR

This paper establishes upper limits on the growth of massive black holes at high redshift using cosmic X-ray background measurements, suggesting black hole growth may be limited or occurs in rare galaxies.

Contribution

It provides the first firm upper bounds on black hole accretion at z>5 based on unresolved X-ray background, highlighting potential constraints on early black hole growth models.

Findings

Maximum accreted mass density at z>5 is < 1.4E4 M_sun Mpc^{-3}.

Including lower-z AGNs reduces the limit to < 0.66E4 M_sun Mpc^{-3}.

Implication that black holes are rare or accretion is efficient only in rare galaxies at high redshift.

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 rho_acc < 1.4E4 M_sun Mpc^{-3} for z>5. Considering the contribution of lower-z AGNs, the value reduces to rho_acc < 0.66E4 M_sun Mpc^{-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 efficient only for black holes hosted by rare galaxies.