AGNs in post-mergers from the ultraviolet near infrared optical northern survey

Abstract

The kinematic disturbances associated with major galaxy mergers are known to produce gas inflows, which in turn may trigger accretion onto the supermassive black holes (SMBH) of the participant galaxies. While this effect has been studied in galaxy pairs, the frequency of active galactic nuclei (AGN) in fully coalesced post-merger systems is poorly constrained due to the limited size or impurity of extant post-merger samples. Previously, we combined convolutional neural network (CNN) predictions with visual classifications to identify a highly pure sample of 699 post-mergers in deep r-band imaging. In the work presented here, we quantify the frequency of AGN in this sample using three metrics: optical emission lines, mid-infrared (mid-IR) colour, and radio detection of low-excitation radio galaxies (LERGs). We also compare the frequency of AGN in post-mergers to that in a sample of spectroscopically identified galaxy pairs. We find that AGN identified by narrow-line optical emission and mid-IR colour have an increased incidence rate in post-mergers, with excesses of ~4 over mass- and redshift-matched controls. The optical and mid-IR AGN excesses in post-mergers exceed the values found for galaxy pairs, indicating that AGN activity in mergers peaks after coalescence. Conversely, we recover no significant excess of LERGs in post-mergers or pairs. Finally, we find that the [OIII] luminosity (a proxy for SMBH accretion rate) in post-mergers that host an optical AGN is ~0.3 dex higher on average than in non-interacting galaxies with an optical AGN, suggesting that mergers generate higher accretion rates than secular triggering mechanisms.

SDSS-IV MaNGA: The incidence of major mergers in type I and II AGN host galaxies in the DR15 sample

“…Carpineti et al (2012) explored a subset of post-mergers, where a single remnant is in the final stages of relaxation after the merger, finding an important rising of the AGN fraction and suggesting that the AGN phase probably becomes dominant only in the very final stages of the merger process. Bickley et al (2023) quantified the frequency of AGN in fully coalesced post-merger systems, and further compared this frequency to that in a sample of galaxy pairs. They find that AGN identified by narrow-line optical emission and mid-IR colour have an incidence rate in post-mergers with excesses of ∼4 over control samples, also exceeding the values found for galaxy pairs, indicating that AGN activity in mergers peaks after coalescence.…”

“…There is compelling evidence showing that merger-induced AGN activity is more important at more advanced stages of the merger process (Ellison et al 2013;Van Wassenhove et al 2012;Bickley et al 2023). High-resolution hydrodynamical simulations predict that AGN activity generally increases with more advanced merger-stages but particularly as a pair coalesces into a post-merger stage (Van Wassenhove et al 2012;Capelo et al 2017).…”

Prevalence of Compact Nuclear Radio Emission in Post-merger Galaxies and Its Origin

“…Among the evolutionary stages of merger systems, postmerger galaxies, those in which the stellar nuclei have coalesced, perhaps present the most unique laboratories to study these triggering mechanisms and the effects of AGN feedback on star formation as a result of the advanced stage of the merger. Small samples of post-merger systems have found hints at enhancement in the star formation rate (SFR; Ellison et al 2013) and AGN incidence over galaxies in close pairs (Carpineti et al 2012;Bickley et al 2023). The effectiveness of AGN feedback, however, is questioned when examining postmerger galaxies.…”