Studying the AGN-Merger Connection through Visual Classifications of CANDELS Galaxies

Mergers play a vital role in galaxy evolution, having the potential to trigger Active Galactic Nucleus (AGN) activity, star formation, or changes in morphology. In this work, we investigate whether galaxy mergers have an effect on AGN activity in the galaxies involved. We used a visual classification scheme to classify the morphologies of nearly 50,000 galaxies and identify galaxy mergers in the CANDELS fields, a Multi-Cycle Treasury Program using the Hubble Space Telescope. We use IR and X-ray selection criteria to identify IR and X-ray AGN, and matched controls to both our AGN subsample and merger subsample. We investigated the fraction of mergers that host AGN, as well as the fraction of AGN that are found in merging systems, to study the AGN-merger connection from multiple angles. We also investigated the properties of mergers, such as the mass ratios of early-stage mergers, and their star forming properties. We find that galaxies involved in a merger or interaction are more likely than non-merging galaxies to host AGN. The fraction of merging systems that host AGN is 0.034±0.002,compared to an AGN fraction of 0.023±0.001 in non-merging systems. We also find that IR AGN are more likely to be found in merging systems, with a merger fraction of 0.353+0.021−0.019, compared to a merger fraction of 0.225+0.007−0.006 in control galaxies without AGN activity. This is likely because these are more dusty and obscured, and are expected to be observed earlier in the merger process, when merger signatures are more visible. The AGN detected in either the X-ray or both the IR and X-ray are even less likely to be found in merging systems, as the merger signatures are expected to fade later in the merging process. These results show that mergers play an important role in driving AGN activity.