The X-ray view of merger-induced active galactic nuclei activity at low redshift

Abstract

ABSTRACT Galaxy mergers are predicted to trigger accretion on to the central supermassive black holes, with the highest rates occurring during final coalescence. Previously, we have shown elevated rates of both optical and mid-IR selected active galactic nuclei (AGNs) in post-mergers, but to date the prevalence of X-ray AGNs has not been examined in the same systematic way. We present XMM–Newton data of 43 post-merger galaxies selected from the Sloan Digital Sky Survey along with 430 non-interacting control galaxies matched in stellar mass, redshift, and environment in order to test for an excess of hard X-ray (2–10 keV) emission in post-mergers attributable to triggered AGNs. We find two X-ray detections in the post-mergers ($4.7^{+9.3}_{-3.8}{{\ \rm per\ cent}}$) and nine in the controls ($2.1^{+1.5}_{-1.0}{{\ \rm per\ cent}}$), an excess of $2.22^{+4.44}_{-2.22}$, where the confidence intervals are 90 per cent. While, we therefore do not find statistically significant evidence for an X-ray AGN excess in post-mergers (p = 0.26), we find a factor of ∼17 excess of mid-IR AGNs in our sample, consistent with the past work and inconsistent with the observed X-ray excess (p = 2.7 × 10−4). Dominant, luminous AGNs are therefore more frequent in post-mergers, and the lack of a comparable excess of 2–10 keV X-ray AGNs suggests that AGNs in post-mergers are more likely to be heavily obscured. Our results are consistent with the post-merger stage being characterized by enhanced AGN fueling, heavy AGN obscuration, and more intrinsically luminous AGN, in line with theoretical predictions.