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Description
Massive black hole (MBH) seeds at redshift z>~10 are now thought to be key ingredients to explain the presence of the supermassive (10^9-10^M_{sun}_) black holes in place <1Gyr after the big bang. Once formed, massive seeds grow and emit copious amounts of radiation by accreting the left-over halo gas; their spectrum can then provide crucial information on their evolution. By combining radiation-hydrodynamic and spectral synthesis codes, we simulate the time-evolving spectrum emerging from the host halo of a MBH seed with initial mass 10^5^M_{sun}_, assuming both standard Eddington-limited accretion, or slim accretion discs, appropriate for super-Eddington flows. The emission occurs predominantly in the observed infrared-submm (1-1000{mu}m) and X-ray (0.1-100keV) bands. Such signal should be easily detectable by JWST around ~1{mu}m up to z~25, and by ATHENA (between 0.1 and 10keV, up to z~15). Ultra-deep X-ray surveys like the Chandra Deep Field South could have already detected these systems up to z~15. Based on this, we provide an upper limit for the z>~6 MBH mass density of {rho}{blackdot}<~2.5x10^2^M_{sun}_/Mpc^3^ assuming standard Eddington-limited accretion. If accretion occurs in the slim disc mode the limits are much weaker, {rho}{blackdot}<~7.6x10^3^M_{sun}_/Mpc^3^ in the most constraining case.
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