The X-ray View of Merger-Induced AGN Activity at Low Redshift
Abstract
Galaxy mergers are predicted to trigger accretion onto the central supermassive black hole, with the highest rates occurring during final coalescence in major, gas-rich mergers. In previous work, we have shown elevated rates of both optical and mid-IR selected active galactic nuclei (AGN) in recent post-mergers, but to date the prevalence of X-ray AGN has not been examined in the same systematic way. In this paper, we present X-ray of 49 post-merger galaxies along with non-interacting control galaxies matched in stellar mass and redshift, in order to test for an excess of X-ray emission from post-merger galaxies attributable to triggered AGN activity. We find that post-mergers exhibit a factor of ~5 excess of X-ray sources with observed 2-10 keV luminosities above 1040 erg/s compared to the controls, and this excess holds to higher X-ray luminosity thresholds, consistent with an enhancement of AGN activity. Mid-IR AGN in the same sample of post-mergers, however, exhibit an excess of ~10 or more, suggesting that the X-ray emission is heavily attenuated. Optical AGN in the same post-mergers show the lowest enhancement over the controls, a factor of about ~2 in agreement with past work, but their observed X-ray luminosities are higher by a factor of ~30 over the X-ray luminosities of post-mergers not classified as optical AGN, suggesting that optically-selected AGN in post-mergers are intrinsically more luminous than non-optical AGN. The difference in optical and X-ray selected AGN excesses can be understood as enhanced star-formation in post-mergers precluding optical AGN classification at lower intrinsic luminosities due to significant contamination of the emission line fluxes from star formation, so that many galaxies classified as star forming from optical emission lines actually host AGN. Post mergers show an enhancement in AGN classified using multiple different criteria, suggesting that their AGN are intrinsically more luminous than AGN in non-mergers. Our results are consistent with the post-merger stage being characterized by both enhanced AGN fueling and heavy AGN obscuration, in line with theoretical predictions.
- Publication:
-
American Astronomical Society Meeting Abstracts #235
- Pub Date:
- January 2020
- Bibcode:
- 2020AAS...23530408S