MeV blazars are the most luminous persistent sources in the Universe and emit
most of their energy in the MeV band. These objects display very large jet
powers and accretion luminosities and are known to host black holes with a mass
often exceeding 109M⊙. An MeV survey, performed by a new generation
MeV telescope which will bridge the entire energy and sensitivity gap between
the current generation of hard X-ray and gamma-ray instruments, will detect
>1000 MeV blazars up to a redshift of z=5−6. Here we show that this would
allow us: 1) to probe the formation and growth mechanisms of supermassive black
holes at high redshifts, 2) to pinpoint the location of the emission region in
powerful blazars, 3) to determine how accretion and black hole spin interplay
to power the jet.Comment: 7 pages, 4 figure. Submitted to the Astro2020 call for Science White
Paper