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

TL;DR

This study demonstrates that post-merger galaxies have a significantly higher fraction of active galactic nuclei (AGN) across multiple wavelengths compared to non-interacting galaxies, supporting the idea that mergers trigger AGN activity.

Contribution

It provides the first comprehensive multiwavelength analysis of AGN activity in a large, volume-limited sample of post-merger galaxies, highlighting the importance of deep X-ray observations.

Findings

Post-mergers have an AGN fraction of 55.7% in X-ray, higher than 23.6% in controls.

Multi-wavelength AGN fraction in post-mergers is 76.6%, compared to 39.1% in controls.

Most optical, IR, and radio AGN are also X-ray AGN, but many X-ray AGN lack other diagnostics.

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_sun > 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_sun > 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.