Do we expect most AGN to live in discs?

Abstract

Recent observations have indicated that a large fraction of the low to intermediate luminosity AGN population lives in disk-dominated hosts, while the more luminous quasars live in bulge-dominated hosts (that may or may not be major merger remnants), in conflict with some previous model predictions. We therefore build and compare a semi-empirical model for AGN fueling which accounts for both merger and non-merger "triggering." In particular, we show that the "stochastic accretion" model -in which fueling in disk galaxies is essentially a random process arising whenever dense gas clouds reach the nucleusprovides a good match to the present observations at low/intermediate luminosities. However it falls short of the high-luminosity population. We combine this with models for major merger-induced AGN fueling, which lead to rarer but more luminous events, and predict the resulting abundance of disk-dominated and bulge-dominated AGN host galaxies as a function of luminosity and redshift. We compile and compare observational constraints from z ∼ 0 − 2. The models and observations generically show a transition from disk to bulge dominance in hosts near the Seyfert-quasar transition, at all redshifts. "Stochastic" fueling dominates AGN by number (dominant at low luminosity), and dominates BH growth below the "knee" in the present-day BH mass function ( 10 7 M ). However it accounts for just ∼ 10% of BH mass growth at masses 10 8 M . In total, fueling in disky hosts accounts for ∼ 30% of the total AGN luminosity density/BH mass density. The combined model also accurately predicts the AGN luminosity function and clustering/bias as a function of luminosity and redshift; however, we argue that these are not sensitive probes of BH fueling mechanisms.

The host galaxies of X-ray selected active galactic nuclei toz= 2.5: Structure, star formation, and their relationships from CANDELS andHerschel/PACS

“…If AGN fueling at high redshifts is primarily modulated by small scale processes, then how do we understand the higher Sérsic, rounder light profiles for AGNs in such high redshift hosts? If this is primarily due to widespread contamination of these profiles by reddened nuclear light, and the true host structures are indeed disk-like and consistent with other massive galaxies, then a strong case can be made that secular fueling is the primary mode at z ∼ 2 (Schawinski et al 2011;Kocevski et al 2012;Rosario et al 2012;Hopkins et al 2014b). However, if there is a preference for AGNs to be found in bulgy galaxies (as energetic arguments seem to support), then the violent mode responsible for the formation of such bulges may still retain a critical role in fueling higher redshift AGNs.…”

“…Therefore, any consideration for AGN triggering scenarios must be mixed, with both secular and violent components. Theoretical insight suggests that this mix evolves with redshift in the low and moderate luminosity AGN population (Hopkins et al 2014b). …”

Mid-infrared luminous quasars in the GOODS–Herschelfields: a large population of heavily obscured, Compton-thick quasars atz≈ 2

“…Di Matteo et al 2005;Hopkins et al 2006), especially at high redshifts (e.g. Hopkins, Kocevski & Bundy 2014). On the other hand, secular processes, such as disc instabilities and galaxy bars, are expected to be the main mechanisms driving the gas into the centre of smaller, lowluminosity systems, where the SMBH and the galaxy experience a slower and more steady growth (see also Alexander & Hickox 2012, for a review).…”

Buried AGNs in Advanced Mergers: Mid-infrared Color Selection as a Dual AGN Candidate Finder

“…Theory predicts that these interactions funnel gas toward the central regions of galaxies (Mihos & Hernquist 1996), potentially triggering gas accretion onto the central supermassive black hole (SMBH) and causing it to shine brightly as an active galactic nucleus (AGN). Although a minority of AGNs by number appear to be hosted in ongoing mergers at both low ( < z 1) (e.g., Cisternas et al 2011;Villforth et al 2014) and high (z 2) redshift (e.g., Schawinski et al 2011;Kocevski et al 2012;Fan et al 2014;Mechtley et al 2016;Villforth et al 2016), observations and semi-empirical modeling suggest that merger-triggered AGNs may dominate SMBH growth, especially at the highest luminosities (e.g., Treister et al 2012;Hopkins et al 2014). Also, as the vast majority of galaxies are thought to contain SMBHs, a direct consequence of the hierarchical model of galaxy formation should be the existence of gravitationally bound binary AGNs, the spatially resolvable precursors of which would be dual AGNs with separations of a few kiloparsecs.…”