2015MNRAS.454.3771P

Massive black hole (MBH) seeds at redshift z ≳ 10 are now thought to be key ingredients to explain the presence of the supermassive (109-10M_☉) 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 10⁵M_☉, assuming both standard Eddington-limited accretion, or slim accretion discs, appropriate for super-Eddington flows. The emission occurs predominantly in the observed infrared-submm (1-1000µm) and X-ray (0.1-100keV) bands. Such signal should be easily detectable by JWSTaround ∼ 1µm 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 ρø ≲ 2.5x10²M_☉/Mpc3 assuming standard Eddington-limited accretion. If accretion occurs in the slim disc mode the limits are much weaker, ρø ≲ 7.6x10³M_☉/Mpc3 in the most constraining case.