The importance of galaxy interactions in triggering type II quasar activity

Abstract

We present deep Gemini Multi‐Object Spectrograph‐South optical broad‐band images for a complete sample of 20 Sloan Digital Sky Survey selected type II quasars taken from Zakamska et al., with redshifts in the range 0.3 < z < 0.41 and [O iii]λ5007 emission‐line luminosities L[O iii ]>108.5L. The images were taken with the aim of investigating the interaction status of the quasar host galaxies, in order to determine the significance of galaxy interactions in triggering nuclear activity. We find that 15 of our sample of 20 (75 per cent) show evidence for interaction in the form of tails, shells, fans, irregular features, amorphous haloes and double nuclei. The median surface brightness of the features is trueμ̃r corr =23.4magarcsec−2 and the range is Δμr corr ≃[20.9,24.7]magarcsec−2. We find a similar rate of interaction signatures in the type II quasars as in a comparison sample of quiescent early‐type galaxies at similar redshift (67 per cent) taken from Ramos Almeida et al. (RA11). However the surface brightness of the detected features is up to 2 mag brighter for the type II quasars than for the quiescent early types, which have surface brightnesses in the range Δμr corr ≃[22.1,26.1]magarcsec−2 and a median surface brightness trueμ̃r corr =24.3magarcsec−2. Despite the relatively small sample size, this may indicate that the mergers witnessed in the comparison sample galaxies could have different progenitors, or we may be viewing the interactions at different stages. We also compare our results with those of Ramos Almeida et al. (RA12) who made a similar analysis using a complete sample of radio‐loud active galactic nuclei (AGN). They find a higher rate of interaction signatures in the radio‐loud AGN (95 per cent) than the type II quasars, but a very similar range of surface brightnesses for the morphological features Δμr corr ≃[20.9,24.8]magarcsec−2, possibly indicating a similarity in the types of triggering interactions. The wide range of features detected in the type II quasar sample suggests that AGN activity can be triggered before, during or after the coalescence of the black holes, with six of the 20 objects (30 per cent) having double nuclei. Overall, the results presented here are consistent with the idea that galaxy interaction plays an important role in the triggering of quasar activity. We also use time‐scale arguments to show that it is unlikely that most radio‐quiet quasars cycle through a radio‐loud phase as part of a single quasar triggering event.

Spatially Offset Active Galactic Nuclei. II. Triggering in Galaxy Mergers

“…Several studies have placed estimates on the fraction of AGNs hosted by galaxies in mergers or merger-remnants (Georgakakis et al 2009;Cisternas et al 2011;Ellison et al 2011;Schawinski et al 2011Schawinski et al , 2012Silverman et al 2011;Bessiere et al 2012;Kocevski et al 2012;Simmons et al 2012;Treister et al 2012;Villforth et al 2014). However, these studies have not estimated the fractions for the specific scenarios of offset AGNs or dual AGNs because the methods by which those studies selected galaxy mergers and cross-matched with AGNs do not uniformly allow for a distinction between systems in which one or both galaxies hosts an AGN.…”

Galaxy And Mass Assembly (GAMA): A “No Smoking” Zone for Giant Elliptical Galaxies?

“…A number of studies found an increased fraction of AGNs in galaxies with close matches, ongoing mergers, or in postmerger systems (Ramos Almeida et al 2011;Ellison et al 2011;Bessiere et al 2012;Cotini et al 2013;Sabater et al 2013). However, several studies claimed to find no significant excess of mergers in AGN hosts (Dunlop et al 2003;Sanchez et al 2004b;Grogin et al 2005;Li et al 2008;Gabor et al 2009;Tal et al 2009;Cisternas et al 2011;Kocevski et al 2012;Schawinski et al 2012;Bohm et al 2013).…”

What Distinguishes the Host Galaxies of Radio-loud and Radio-quiet AGNs?

“…It was claimed that the most radio-loud AGNs are preferentially hosted by bulgedominated galaxies with masses larger than 10 11 M ⊙ and BH masses larger than 10 8 M ⊙ (e.g. Laor 2000; Dunlop et al 2003;Floyd et al 2004;McLure & Jarvis 2004;Best et al 2005); the fraction of galaxies with disturbed morphology is larger in RL AGNs than in RQ AGNs (Bessiere et al 2012;Chiaberge et al 2015); the star-formation rate (SFR) in the hosts of RL AGNs is lower than in the hosts of RQ AGNs (Dicken et al 2012;Floyd et al 2013); the environment is denser around RL AGNs than around RQ AGNs (Mandelbaum et al 2009;Shen et al 2009;Donoso et al 2010;Falder et al 2010;Ramos Almeida et al 2013). The dependence of radio loudness on host-galaxy properties is also indicated by studies based on samples selected from massive optical and radio surveys (Kauffmann et al 2008;Best & Heckman 2012;Gürkan et al 2015).…”