Limits on the high redshift growth of massive black holes

¹ INAF, IASF Milano, via E. Bassini 15, 20133 Milano, Italy
e-mail: haardt@mib.infn.it
² DiSAT, Università dell’Insubria, via Valleggio 11, 22100 Como, Italy
³ INFN, Sezione di Milano-Bicocca, Piazza delle Scienze 3, 20123 Milano, Italy
⁴ IAP, 98bis Boulevard Arago, 75014 Paris, France
⁵ INAF, Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy

Received: 6 July 2012
Accepted: 3 August 2012

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 × 10⁴   M_⊙ Mpc^-3 for z ≳ 5. Considering the contribution of lower-z AGNs, the value reduces to ρ_acc ≲ 0.66 × 10⁴   M_⊙ 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 only efficient for the black holes hosted by rare galaxies.