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