A Multiwavelength Study of Active Galactic Nuclei in Post-Merger Remnants

Abstract

We investigate the role of galaxy mergers in triggering AGN in the nearby Universe. Our analysis is based on a sample of 79 post-merger remnant galaxies with deep X-ray observations from Chandra/XMM-Newton capable of detecting a low-luminosity AGN of ≥ 10 40.5 erg s −1 . This sample is derived from a visually classified, volume-limited sample of 807 post-mergers identified in the Sloan Digital Sky Survey Data Release 14 with log M * /M ≥ 10.5 and 0.02 ≤z≤ 0.06. We find the X-ray AGN fraction in this sample is 55.7%±5.6% compared to 23.6%±2.8% for a mass and redshift matched non-interacting control sample. The multi-wavelength AGN fraction (identified as an AGN in one of X-ray, IR, radio or optical diagnostics) for post-mergers is 76.6% ± 4.8% compared to 39.1% ± 3.2% for controls. Thus post-mergers exhibit a high overall AGN fraction with an excess between 2 -4 depending on the AGN diagnostics used. In addition, we find most optical, IR, and radio AGN are also identified as X-ray AGN while a large fraction of X-ray AGN are not identified in any other diagnostic. This highlights the importance of deep X-ray imaging to identify AGN. We find the X-ray AGN fraction of post-mergers is independent of stellar mass above log M * /M ≥ 10.5 unlike the trend seen in control galaxies. Overall, our results show that post-merger galaxies are a good tracer of the merger-AGN connection and strongly support the theoretical expectations that mergers trigger AGN.