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Shining in the dark: the spectral evolution of the first black holes

Abstract : Massive black hole (MBH) seeds at redshift z ≳ 10 are now thought to be key ingredients to explain the presence of the supermassive (109-10 M) black holes in place <1 Gyr 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 105 M, 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 μm) and X-ray (0.1-100 keV) bands. Such signal should be easily detectable by JWSTaround ∼ 1 μm up to z ∼ 25, and by ATHENA (between 0.1 and 10 keV, 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 ρ ≲ 2.5 × 102 M Mpc-3 assuming standard Eddington-limited accretion. If accretion occurs in the slim disc mode the limits are much weaker, ρ ≲ 7.6 × 103 M Mpc-3 in the most constraining case.
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Fabio Pacucci, Andrea Ferrara, Marta Volonteri, Guillaume Dubus. Shining in the dark: the spectral evolution of the first black holes. Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2015, 454, pp.3771-3777. ⟨10.1093/mnras/stv2196⟩. ⟨insu-03644669⟩



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