Do AGN triggering mechanisms vary with radio power? – II. The importance of mergers as a function of radio power and optical luminosity

Abstract

Investigation of the triggering mechanisms of radio AGN is important for improving our general understanding of galaxy evolution. In the first paper in this series, detailed morphological analysis of high-excitation radio galaxies (HERGs) with intermediate radio powers suggested that the importance of triggering via galaxy mergers and interactions increases strongly with AGN radio power and weakly with optical emission-line luminosity. Here, we use an online classification interface to expand our morphological analysis to a much larger sample of 155 active galaxies (3CR radio galaxies, radio-intermediate HERGs and Type 2 quasars) that covers a broad range in both 1.4 GHz radio power and [OIII]λ5007 emission-line luminosity. All active galaxy samples are found to exhibit excesses in their rates of morphological disturbance relative to 378 stellar-mass- and redshift-matched non-active control galaxies classified randomly and blindly alongside them. These excesses are highest for the 3CR HERGs (4.7 σ) and Type 2 quasar hosts (3.7 σ), supporting the idea that galaxy mergers provide the dominant triggering mechanism for these subgroups. When the full active galaxy sample is considered, there is clear evidence to suggest that the enhancement in the rate of disturbance relative to the controls increases strongly with [OIII]λ5007 emission-line luminosity but not with 1.4 GHz radio power. Evidence that the dominant AGN host types change from early-type galaxies at high radio powers to late-type galaxies at low radio powers is also found, suggesting that triggering by secular, disk-based processes holds more importance for lower-power radio AGN.

Warm molecular and ionized gas kinematics in the type-2 quasar J0945+1737

“…1), but it is clearly visible in the deeper image obtained with the Wide Field Camera (WFC) on the Isaac Newton Telescope (INT), at the Roque de Los Muchachos Observatory, in La Palma (see right panel of Fig. 1 and Pierce et al 2022). There is also a shorter tidal tail extending in the opposite direction, to the south-east.…”

“…The morphological signatures of mergers are usually studied by looking at the stellar continuum (e.g. Pierce et al 2022). Thus, the discrepant red voxel most likely corresponds to tidally disrupted gas associated with the merger or interaction that produced the disturbed optical morphology, both on the small-scales probed by the NIFS data, and on the large-scale shown in Fig.…”

Quantifying the cool ISM in radio AGNs: evidence for late-time retriggering by galaxy mergers and interactions

“…As they are almost invariably hosted in elliptical galaxies (e.g. Tadhunter 2016 ;Pierce et al 2022 ), they offer a clean way to search for signs of interactions and investigate triggering and feedback mechanisms (see Section 1 ).…”

Compact radio sources: Triggering and feedback

“…Overall, based on studies of optical morphologies, it is likely that galaxy mergers and interactions are the dominant triggering mechanism for powerful radio sources in the local universe with strong nuclear emission‐line activity, including the CSS/GPS sources (Pierce et al 2021; Ramos Almeida et al 2011, 2012). However, from the evidence of their SFR and dust masses, it appears that CSS/GPS sources are triggered in mergers that are more major and/or gas‐rich than those that trigger extended radio sources.…”