Cosmological simulations of black hole growth II: how (in)significant are merger events for fuelling nuclear activity?

Abstract

Which mechanism(s) are mainly driving nuclear activity in the centres of galaxies is a major unsettled question. In this study, we investigate the statistical relevance of galaxy mergers for fuelling gas onto the central few kpc of a galaxy, potentially resulting in an active galactic nucleus (AGN). To robustly address that, we employ large-scale cosmological hydrodynamic simulations from the Magneticum Pathfinder set, including models for BH accretion and AGN feedback. Our simulations predict that for luminous AGN (L AGN > 10 45 erg/s) at z = 2, more than 50 per cent of their host galaxies have experienced a merger in the last 0.5 Gyr. These high merger fractions, however, merely reflect the intrinsically high merger fractions of massive galaxies at z = 2, in which luminous AGN preferentially occur. Apart from that, our simulations suggest that merger events are not the statistically dominant fuelling mechanism for nuclear activity over a redshift range z = 0−2: irrespective of AGN luminosity, less than 20 per cent of AGN hosts have on average undergone a recent merger, in agreement with a number of observational studies. The central ISM conditions required for inducing AGN activity can be, but are not necessarily caused by a merger. Despite the statistically minor relevance of mergers, at a given AGN luminosity and stellar mass, the merger fractions of AGN hosts can be by up to three times higher than that of inactive galaxies. Such elevated merger fractions still point towards an intrinsic connection between AGN and mergers, consistent with our traditional expectation.

Galaxy mergers in eagle do not induce a significant amount of black hole growth yet do increase the rate of luminous AGN

“…However, by these times merging systems are now so rare that their (albeit enhanced) contribution is still not highly significant, and isolated galaxies remain the dominant source of BH accretion at lower redshifts. The conclusion of mergers never being statistically relevant fuelling mechanisms for BHs is consistent with the results from the Magneticum Pathfinder simulation, who performed a similar analysis to this study in the high-mass regime (M * ≥ 10 11 M , Steinborn et al 2018). However we emphasise that if we were to of restricted our study to just the high-mass regime (M * ≥ 10 11 M ) as they did, we would not of found the same result (see Figure 4).…”

“…Hydrodynamical simulations of merging systems have provided compelling theoretical evidence for a link between BH activity and galaxy interactions (e.g., Dubois et al 2015;Pontzen et al 2017), yet the global significance of the merging process for boosting BH activity within a full cosmological context remains largely unknown. Steinborn et al (2018) investigated the role of galaxy mergers as driving mechanisms for BH activity in the high mass regime (M * ≥ 10 11 M ) using the cosmological hydrodynamical Magneticum Pathfinder simulations. They argue, that whilst the merger fractions of AGN hosts can be up to three times higher than those of inactive galaxies, the role of merg-ers in high-mass galaxies are not statistically relevant for BH fuelling.…”

Dark-ages reionization and galaxy formation simulation – XVIII. The high-redshift evolution of black holes and their host galaxies

“…The hydrodynamical simulation Horizon-AGN found that only ∼ 35 per cent of black hole mass in local massive galaxies is directly attributable to merging, with the majority of black hole growth instead growing via secular processes (Martin et al 2018). The Magneticum Pathfinder Simulation also found that merger events are not the dominant fuelling mechanism for black holes in z = 0-2, with merger fractions less than 20 per cent, except for very luminous quasars at z 2 (Steinborn et al 2018).…”

Tracing black hole and galaxy co-evolution in the Romulus simulations

“…Mergers are associated with AGN in the EAGLE cosmological simulations, with a more compelling link at low-redshift, perhaps because gas is less readily available at late times (McAlpine et al 2018). Yet analyzing the Magneticum pathfinder simulations, Steinborn et al (2018) find that mergers only play a minor role in triggering AGN. In these simulations, the association of the most luminous AGN with mergers can to some extent be explained by the fact that more massive galaxies are inherently more likely to be in merging systems.…”