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

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 χ 2 test with respect to the observed AGN hard X-ray luminosity function. The observed black hole space density, AGN number counts, and Xray 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 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.

Do we expect most AGN to live in discs?

“…We reach similar conclusions to those recently reached by Draper & Ballantyne (2012), using an independent BH population synthesis approach with very different methods used to model the merger and non-merger triggering rates. In short, the models predict that "stochastic" fueling, with no specific preference for largescale "triggering phenomena" in disky, secularly evolving systems should dominate the population at Seyfert and lower luminosities, while mergers dominate fueling of bright quasars.…”

“…This includes the fact that quasars exhibit excessive small-scale (sub-halo scale) clustering while Seyferts do not (Serber et al 2006;Myers et al 2007b;Serber et al 2006;Hennawi et al 2009;Shen et al 2010); future observations should confirm that this difference appears even considering bulge-dominated galaxies. Quasar duty cycles rise more sharply with redshift (in agreement with observed merger rates), as opposed to Seyfert duty cycles which increase more slowly more or less in agreement with galaxy gas fraction evolution (see the compilation in and discussion in Draper & Ballantyne 2012); this is qualitatively opposite the trend in galaxy bulge-to-disk ratios (galaxies become less bulge-dominated, even at fixed mass, at high redshift) -it is therefore very difficult to predict a trend purely from secular or stochastic fueling in which the ratio of disk-to-bulge dominance is nearly redshift-independent (as we predict here). We also expect a much larger prevalence of "post-starburst" (or recently starforming, K+A or E+A type) populations in true quasars compared specifically to normal bulges (not disks) of the same mass, as many observations have suggested (Brotherton et al 1999 Nandra et al 2007;Silverman et al 2008;Higdon et al 2008;Glikman et al 2012;Cales et al 2013).…”

“…One consequence of such models is the idea (discussed in detail in Draper & Ballantyne 2012;Santini et al 2012;Treister et al 2012) that there is some characteristic host bulge/BH mass (and corresponding quasar luminosity) below which these more ubiquitous mechanisms dominate AGN fueling (being more common and requiring less bulge growth to deepen the central potential in this mass regime). Above this division, less violent mechanisms are simply inefficient (they may still happen, but they do not sufficiently raise the bulge mass, so BHs quickly self-regulate and do not experience any significant lifetime of high-Eddington ratio growth) and the population requires more extreme mechanisms such as major mergers to build the most massive bulges and (corresponding) BHs.…”

Selection bias in dynamically measured supermassive black hole samples: its consequences and the quest for the most fundamental relation

“…Moreover, our results suggest that (the bulges of) spirals, which are usually not considered to have undergone a substantial phase of late dry mergers (e.g., Huertas-Company et al 2013;Patel et al 2013;Huertas-Company et al 2015), define similar correlations as do ellipticals, thus further pointing to the M bh -σ relation as the dominant correlation. This weakens the motivation for models in which M bh in spirals grows substantially via any secular process unrelated to σ (e.g., Bower et al 2006;Hopkins & Hernquist 2009;Bournaud et al 2011;Draper & Ballantyne 2012;Shankar et al 2012b;Fontanot et al 2015;Gatti et al 2016).…”

The cosmic growth of the active black hole population at 1 <z <2 in zCOSMOS, VVDS and SDSS

“…Boyle et al 2000;Wolf et al 2003;Ueda et al 2003;Hasinger et al 2005;La Franca et al 2005;Richards et al 2006a;Bongiorno et al 2007;Silverman et al 2008;Croom et al 2009;Aird et al 2010;Assef et al 2011;Fiore et al 2012;Ueda et al 2014). The evolution of the AGN LF allows us to constrain the BH growth history using a continuity equation to evolve the black hole mass function (BHMF) through redshift (Yu & Tremaine 2002;Yu & Lu 2004;Marconi et al 2004;Merloni 2004;Merloni & Heinz 2008;Yu & Lu 2008;Shankar et al 2009;Draper & Ballantyne 2012). These studies found that the local total BHMF is consistent with being a relic of previous AGN activity.…”