Multiflavour SMBH seeding and evolution in cosmological environments

Abstract

ABSTRACT We study the genesis and evolution of supermassive black hole (SMBH) seeds through different formation channels, from PopIII remnants to massive seeds, modelled within the L-Galaxies semi-analytic code. We run the model on the Millennium-II simulation (MR-II) merger trees, as their halo-mass resolution ($M_{\rm vir,res} \sim 10^7\, \mathrm{M}_\odot \, h^{-1}$) allows to study in a cosmological volume ($L_{\rm box}=100\, \mathrm{Mpc}\, h^{-1}$) the evolution of atomic-cooling haloes ($T_{\rm vir} \gtrsim 10^4\, \mathrm{K}$) where intermediate-mass and heavy seeds are expected to form. We track the formation of these seeds according to spatial variations of the chemical and radiative feedback of star formation. Not being able to resolve the first mini-haloes ($T_{\rm vir} \sim 10^3\, \mathrm{K}$), we inherit evolved PopIII remnants in a sub-grid fashion, using the results of the GQd model. We also include the formation of heavy seeds in gas-rich massive mergers, which are very rare in the MR-II volume. The descendants of light seeds numerically prevail among our SMBHs population at all masses and z. Heavier seeds form in dense environments where close neighbours provide the required UV illumination. Overall, our model produces a z = 0 SMBHs population whose statistical properties meet current constraints. We find that the BH occupation fraction highly depends on the seeding efficiency and that the scaling relation between BH and stellar mass, in the dwarf-mass regime, is flatter than in the high-mass range. Finally, a fraction of BHs hosted in local dwarf galaxies never grow since they form at z > 6.