The origin of supermassive black holes at cosmic dawn

ABSTRACT We investigate the steady spherically symmetric accretion in the combined potential of a central black hole and a dark matter halo. For the halo, we consider a Hernquist and an NFW potential and calculate the critical points of the flow. We find that the trans-sonic solution to the centre is not possible without a black hole, whereas two types of trans-sonic solutions are possible in its presence. We also derive the mass accretion rate for a black hole at the centre of a dark matter halo. Our results indicate two phases of accretion. The first is an initial phase with a low accretion rate that depends on the black hole mass, followed by a second phase with a high accretion rate that depends on the halo mass. In the second phase, the black hole mass increases rapidly to supermassive scales, which explains the existence of quasars at redshift z ≥ 6 and also the supermassive black holes (SMBHs) recently detected by the JWST. Further, we calculate the evolution of the Eddington ratio for growing black holes. The accretion is mostly sub-Eddington except for a short super-Eddington episode when the mass accretion rate transitions from low to high. However, during that episode, the black hole mass is likely inadequate to hinder accretion through radiative feedback.