Major Mergers Are Not the Dominant Trigger for High-accretion AGNs at z ∼ 2

Abstract

Research over the past decade has shown diminishing empirical evidence for major galaxy mergers being a dominating or even important mechanism for the growth of supermassive black holes in galaxies and the triggering of optically or X-ray selected active galactic nuclei (AGN).We here for the first time test whether such a connection exists at least in the most 'plausible' part of parameter space for this mechanism: the highest specific accretion rate broad-line AGNs at the peak epoch of black hole activity around z = 2. To that end we examine 21 galaxies hosting a high accreting black hole (L/L edd > 0.7) observed with HST/WFC3 and 92 stellar mass-and redshiftmatched inactive galaxies taken from the CANDELS survey.We removed the AGN point sources from their host galaxies and avoided bias in visual classification by adding and then subtracting mock point sources to and from the comparison galaxies, producing matched residual structures for both sets. The resulting samples were joined, randomized, and subsequently visually ranked with respect to perceived strength of structural distortions by 10 experts. The ensuing individual rankings were combined into a consensus sequence and from this we derived merger fractions for both samples. With the merger fractions f m,agn = 0.24 ± 0.09 for the AGN host galaxy sample and f m,ina = 0.19 ± 0.04 for the inactive galaxies, we find no significant difference between the AGN host galaxies and inactive galaxies. Also, both samples display comparable fractions of diskdominated galaxies. These findings are consistent with previous studies for different AGN populations, and we conclude that even black hole growth at the highest specific accretion rates and at the peak of cosmic AGN activity is not predominantly caused by major mergers.Considering AGN and merger timescales we conclude that it is highly unlikely that a potential time lag could significantly affect our results. We also calculate the inverse view and infer that only ∼20% of all merging galaxies have high-accretion AGN activity during the ongoing merger. Eventually, we also find no significant connection between post-starburst galaxies and AGNs.

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

“…These observations therefore agree with the results presented in Figures 1 and 2, and suggests, both in the observations and the simulation, that mergers are directly responsible for triggering an increased amount of BH activity for at least a subset of the galaxy population. However, these observational results, and therefore our own, then disagree with the empirical studies that find no discernible enhancement in AGN activity around the time of a merger (e.g., Kocevski et al 2012;Schawinski et al 2015;Villforth et al 2017;Marian et al 2019). When it comes to the observed AGN fraction of galaxies with close companions, an increasing excess of AGN with decreasing pair separation has been found (e.g., Ellison et al 2011), further reinforcing the mergers triggering BH activity scenario, and again agreeing with the results from this study (see Figure 3 and also Section 4.2.1).…”

“…However, Schawinski et al (2012) see no such trend for similarly luminous AGN, finding the majority of their host galaxies to be disk dominated, and therefore showing no sign of a recent interaction. Still at high redshift, low and intermediate luminosity AGN (L bol ≤ 10 45 erg s −1 ) typically exhibit merging fractions very similar to that of the inactive population (e.g, Kocevski et al 2012;Schawinski et al 2011;Rosario et al 2015;Mechtley et al 2016;Marian et al 2019), suggesting that mergers have little influence towards enhancing BH activity in this regime.…”

Investigating the Effect of Galaxy Interactions on the Enhancement of Active Galactic Nuclei at 0.5 < z < 3.0

“…While in the nearby universe ∼80% of all quasars (or highluminosity AGNs) show signs of a recent or ongoing merger (Sanders et al 1988a(Sanders et al , 1988bBennert et al 2008;Urrutia et al 2008), our results do not show AGN enhancement even in the highest X-ray luminosity range. Our results are consistent with the results of Marian et al (2019), who consider the highest specific accretion broad line AGNs at the peak epoch of AGN activity around z∼2 and find no significant difference in the merger fraction of the AGN host galaxies and (massand redshiftmatched) non-AGN galaxies. However, Treister et al (2012) find that mergers are responsible for triggering the highest luminosity AGNs at 0<z<3 (z<1 for most of their sample), with no signs of redshift dependence.…”

A meeting at z ∼ 3: Young massive galaxies and an AGN within 30 kpc of the luminous QSO LBQS 0302–0019

“…Examples are local low-metallicity dwarf galaxies (e.g. Kehrig et al 2015Kehrig et al , 2018Senchyna et al 2020) Mechtley et al 2016;Villforth et al 2017;Marian et al 2019), it is still a potential channel for coordinated galaxy and black hole growth as AGN luminosity, star formation, and gas content seem to be linked (e.g. Rosario et al 2013;Husemann et al 2017;Zhuang et al 2021).…”

“…Sanders et al 1988;Hopkins et al 2008), this has been strongly debated in recent years (e.g. Mechtley et al 2016;Villforth et al 2017;Weigel et al 2018;Marian et al 2019). Nevertheless, to confirm the former scenario, a strong interest has been growing to identify so-called dual and binary active galactic nuclei (AGN), where two SMBHs with separations of 100 kpc down to a few parsecs are active simultaneously (see De Rosa et al 2019, for a recent review).…”