Massive Black Hole (MBH) seeds at redshift z≳10 are now thought to
be key ingredients to explain the presence of the super-massive (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 disks,
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 JWST around
∼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 ρ∙≲2.5×102M⊙Mpc−3 assuming standard Eddington-limited accretion. If accretion
occurs in the slim disk mode the limits are much weaker, ρ∙≲7.6×103M⊙Mpc−3 in the most
constraining case.Comment: Submitted for publication in MNRA