Shining in the Dark: the Spectral Evolution of the First Black Holes

Massive Black Hole (MBH) seeds at redshift $z\gtrsim 10$ are now thought to be key ingredients to explain the presence of the super-massive ($10^{9-10} , ${\mathrm{M}}_{\odot }$$)blackholesinplace$ < 1 , $\mathrm{G}\mathrm{y}\mathrm{r}$$ 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 ${10}^5{\mathrm{M}}_{\odot }$, assuming both standard Eddington-limited accretion, or slim accretion disks, appropriate for super-Eddington flows. The emission occurs predominantly in the observed infrared-submm ($1-1000\mu \mathrm{m}$) and X-ray ($0.1 - 100 , $\mathrm{k}\mathrm{e}\mathrm{V}$$)bands\mathrm{.}SuchsignalshouldbeeasilydetectablebyJWSTaround$\sim 1 , $\mu \mathrm{m}$$upto$z \sim 25$,andbyATHENA(between$0.1$and$10 , $\mathrm{k}\mathrm{e}\mathrm{V}$$,upto$z \sim 15$). Ultra-deep X-ray surveys like the Chandra Deep Field South could have already detected these systems up to $z \sim 15$\mathrm{.}Basedonthis,weprovideanupperlimitforthe$z \gtrsim 6$ MBH mass density of $\rho_{\bullet} \lesssim 2.5 \times 10^{2} , \mathrm{M_{\odot} , Mpc^{-3}}$ assuming standard Eddington-limited accretion. If accretion occurs in the slim disk mode the limits are much weaker, $\rho_{\bullet} \lesssim 7.6 \times 10^{3} , ${\mathrm{M}}_{\odot }\mathrm{M}\mathrm{p}{\mathrm{c}}^{-3}$$ in the most constraining case.