The evolution of supermassive blackhole mass--bulge mass relation by a semi-analytic model, $\nu^2$GC

TL;DR

This study uses a semi-analytical model to explore how the relationship between supermassive black hole mass and bulge mass evolves over cosmic time, revealing two distinct growth sequences driven by different galaxy processes.

Contribution

It introduces a semi-analytical model that explains the emergence of two SMBH-bulge mass sequences at high redshift and their convergence in the local universe.

Findings

Two SMBH-bulge mass sequences appear at high redshift.

Galaxy mergers and disc instabilities drive the two sequences.

Dry mergers help unify the sequences into the local relation.

Abstract

We have investigated the redshift evolution of the relationship between supermassive black hole (SMBH) mass and host bulge mass using a semi-analytical galaxy formation model $\nu^2$GC. Our model reproduces the relation in the local universe well. We find that, at high redshift ($z \gtrsim 3$), two sequences appear in the SMBH mass--bulge mass plane. The emergence of these two sequences can be attributed to the primary triggers of the growth of the SMBHs and bulges: galaxy mergers and disc instabilities. The growth of SMBHs and bulges as a result of galaxy mergers is responsible for giving rise to the high-mass sequence, in which SMBHs are more massive for a given host bulge mass than in the low-mas sequence. Conversely, disc instabilities are accountable for the emergence of the low-mass sequence. At lower redshifts, galaxy mergers tend to become increasingly deficient in gas, resulting in a preferential increase of bulge mass without a corresponding growth in SMBH mass. This has the effect of causing galaxies in the upper sequence to shift towards the lower one on the SMBH mass-bulge mass plane. The galaxies that undergo dry mergers serve to bridge the gap between the two sequences, eventually leading to convergence into a single relation known in the local universe. Our results suggest that the observations of the SMBH mass-bulge mass relation in high redshifts can provide insight into their growth mechanisms.