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 Limits on the high redshift growth of massive black holes


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We place firm upper limits on the global accretion history of massive black holes at z5 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 z5. 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 z7 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.



Author: R. Salvaterra; F. Haardt; M. Volonteri; A. Moretti

Source: https://archive.org/



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