Episodic activities of supermassive black holes at redshift $z\le 2$: driven by mergers?

TL;DR

This study models quasar luminosity functions to investigate how galaxy mergers influence supermassive black hole activity over cosmic time, revealing a shift from major to minor mergers as the primary trigger from high to low redshift.

Contribution

It introduces a model linking galaxy merger rates and quasar luminosity functions, showing the evolving role of major and minor mergers in SMBH activity across redshifts.

Findings

Major merger fraction decreases from 0.2 at z~2 to near zero at z~0.

SMBH spins are maximized after major mergers at high redshift.

Minor mergers dominate SMBH activity at low redshift.

Abstract

It has been suggested for quite a long time that galaxy mergers trigger activities of supermassive black holes (SMBHs) on the grounds of imaging observations of individual galaxies. To quantitatively examine this hypothesis, we calculate quasar luminosity functions (LFs) by manipulating the observed galaxy LFs ($z\lesssim 2$) and theoretical merger rates from semi-analytical formulations. We find that the model reproduces the observed quasar LFs provided that the mass ratio ($q$) of the secondary galaxy to the newly formed one changes with cosmic time. The results show that the fraction of major mergers decreases from $f_{\rm maj}\sim 0.2$ at $z\sim 2$ to $f_{\rm maj}\to 0$ at $z\sim 0$. As a consequence, the newly formed SMBHs from major mergers at $z\sim 2$ may acquire a maximal spin due to the orbital angular momentum of the merging holes. Subsequently, random accretion led by minor mergers rapidly drives the SMBHs to spin down. Such an evolutionary trend of the SMBH spins is consistent with that radiative efficiency of accreting SMBHs strongly declines with cosmic time, reported by Wang et al. (2009). This suggests that minor mergers are important in triggering activities of SMBHs at low redshift while major mergers may dominate at high redshift.