A Tale of Two Populations: The Contribution of Merger and Secular Processes to the Evolution of Active Galactic Nuclei

TL;DR

This study uses population synthesis modeling to evaluate the roles of merger and secular processes in triggering active galactic nuclei, revealing that non-merger mechanisms dominate at lower redshifts and influence black hole growth.

Contribution

It introduces a combined observational and theoretical modeling approach to distinguish the contributions of merger and secular processes in AGN evolution.

Findings

Major mergers cannot explain all AGN activity.

Non-merger processes dominate AGN triggering at z 80.5.

Different black hole mass function evolution for merger- and secular-triggered AGN.

Abstract

Due to the co-evolution of supermassive black holes and their host galaxies, understanding the mechanisms that trigger active galactic nuclei (AGN) are imperative to understanding galaxy evolution and the formation of massive galaxies. It is observationally difficult to determine the trigger of a given AGN due to the difference between the AGN lifetime and triggering timescales. Here, we utilize AGN population synthesis modeling to determine the importance of different AGN triggering mechanisms. An AGN population model is computed by combining an observationally motivated AGN triggering rate and a theoretical AGN light curve. The free parameters of the AGN light curve are constrained by minimizing a \chi squared test with respect to the observed AGN hard X-ray luminosity function. The observed black hole space density, AGN number counts, and X-ray background spectrum are also considered as observational constraints. It is found that major mergers are not able to account for the entire AGN population. Therefore, non-merger processes, such as secular mechanisms, must also trigger AGN. Indeed, non-merger processes are the dominant AGN triggering mechanism at z \lesssim 1--1.5. Furthermore, the shape and evolution of the black hole mass function of AGN triggered by major mergers is intrinsically different from the shape and evolution of the black hole mass function of AGN triggered by secular processes.