2015MNRAS.451.1964S

We make black hole (BH) merger trees from Millennium and Millennium-II simulations to find under what conditions 10⁹M_☉ SMBH can form by redshift z = 7. In order to exploit both: large box size in the Millennium simulation; and large mass resolution in the Millennium-II simulation, we develop a method to combine these two simulations together, and use the Millennium-II merger trees to predict the BH seeds to be used in the Millennium merger trees. We run multiple semi-analytical simulations where SMBHs grow through mergers and episodes of gas accretion triggered by major mergers. As a constraint, we use observed BH mass function at redshift z = 6. We find that in the light of the recent observations of moderate super-Eddington accretion, low-mass seeds (100M_☉) could be the progenitors of high-redshift SMBHs (z ∼ 7), as long as the accretion during the accretion episodes is moderately super-Eddington, where fEdd = 3.7 is the effective Eddington ratio averaged over 50 Myr.