SMBH growth parameters in the early Universe of Millennium and Millennium-II simulations

TL;DR

This study combines Millennium and Millennium-II simulations to explore conditions for early supermassive black hole formation, highlighting the role of moderate super-Eddington accretion in growing low-mass seeds to billion-solar-mass SMBHs by redshift 7.

Contribution

It introduces a novel method to merge large-scale and high-resolution simulations and predicts SMBH growth pathways considering recent super-Eddington accretion observations.

Findings

Low-mass seeds can form high-redshift SMBHs with super-Eddington accretion.

Combining simulations improves understanding of SMBH growth conditions.

Moderate super-Eddington accretion (f_Edd=3.7) is sufficient for SMBH formation by z=7.

Abstract

We make black hole (BH) merger trees from Millennium and Millennium-II simulations to find under what conditions 10^9Msun 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 (100Msun) could be the progenitors of high-redshift SMBHs (z~7), as long as the accretion during the accretion episodes is moderately super-Eddington, where f_Edd=3.7 is the effective Eddington ratio averaged over 50 Myr.