Volume 545, September 2012
|Number of page(s)||4|
|Published online||05 September 2012|
Limits on the high redshift growth of massive black holes
INAF, IASF Milano, via E. Bassini 15,
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
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 ρacc ≲ 1.4 × 104 M⊙ Mpc-3 for z ≳ 5. Considering the contribution of lower-z AGNs, the value reduces to ρacc ≲ 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.
Key words: cosmology: observations / X-rays: diffuse background / galaxies: active
© ESO, 2012
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