The X-ray View of Merger-Induced AGN Activity at Low Redshift
  Galaxy mergers are predicted to trigger accretion onto 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 (AGN) in post-mergers, but to date the
prevalence of X-ray AGN 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 AGN. We
find 2 X-ray detections in the post-mergers (4.7^{+9.3}_{-3.8}%) and 9 in the
controls (2.1^{+1.5}_{-1.0}%), an excess of 2.22^{+4.44}_{-2.22}, where the
confidence intervals are 90%. 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 AGN in our sample, consistent with past
work and inconsistent with the observed X-ray excess (p = 2.7 x 10^{-4}).
Dominant, luminous AGN are therefore more frequent in post-mergers, and the
lack of a comparable excess of 2-10 keV X-ray AGN suggests that AGN in
post-mergers are more likely to be heavily obscured. Our results are consistent
with the post-merger stage being characterised by enhanced AGN fueling, heavy
AGN obscuration, and more intrinsically luminous AGN, in line with theoretical
predictions.
