Forming supermassive black holes by accreting dark and baryon matter

Abstract

Given a large-scale mixture of self-interacting dark matter (SIDM) particles and baryon matter distributed in the early Universe, we advance here a two-phase accretion scenario for forming supermassive black holes (SMBHs) with masses around ∼109 M⊙ at high redshifts z(⋆6). The first phase is conceived to involve a rapid quasi-spherical and quasi-steady Bondi accretion of mainly SIDM particles embedded with baryon matter on to seed black holes (BHs) created at redshifts z≲ 30 by the first generation of massive Population III stars; this earlier phase rapidly gives birth to significantly enlarged seed BH masses of during z∼ 20-15, where σ0 is the cross-section per unit mass of SIDM particles and Cs is the velocity dispersion in the SIDM halo referred to as an effective ‘sound speed'. The second phase of BH mass growth is envisaged to proceed primarily via baryon accretion, eventually leading to SMBH masses of MBH∼ 109 M⊙; such SMBHs may form either by z∼ 6 for a sustained accretion at the Eddington limit or later at lower z for sub-Eddington mean accretion rates. In between these two phases, there is a transitional yet sustained diffusively limited accretion of SIDM particles which in an eventual steady state would be much lower than the accretion rates of the two main phases. We intend to account for the reported detections of a few SMBHs at early epochs, e.g. Sloan Digital Sky Survey (SDSS) 1148+5251 and so forth, without necessarily resorting to either super-Eddington baryon accretion or very frequent BH merging processes. Only extremely massive dark SIDM haloes associated with rare peaks of density fluctuations in the early Universe may harbour such early SMBHs or quasars. Observational consequences are discussed. During the final stage of accumulating a SMBH mass, violent feedback in circumnuclear environs of a galactic nucleus leads to the central bulge formation and gives rise to the familiar empirical MBH−σb correlation inferred for nearby normal galaxies with σb being the stellar velocity dispersion in the galactic bulge; in our scenario, the central SMBH formation precedes that of the galactic bulge.