Abstract:
In our current cosmological paradigm, galaxy mergers play an integral in the hierarchical growth in the Universe, while at the same time we now know that most major galaxies host supermassive black holes (SMBHs) at their centers. Extensive theoretical work has shown that galaxy mergers efficiently funnel large quantities of gas into the centers of the galaxies, potentially fueling the SMBHs as active galactic nuclei (AGNs). Dual AGNs, where both SMBHs are active, are predicted to be a critical stage of SMBH growth, coinciding with the peak SMBH growth and the highest obscuring columns of gas. However, the number of confirmed dual AGNs, predicted to be the most frequent in late-stage mergers, remains surprisingly low. Motivated by the fact that dual AGNs in late-stage mergers are predicted to be heavily obscured, we selected a sample of galaxy mergers exhibiting strong mid-IR colors, indicative of powerful, buried AGNs, and obtained high spatial resolution imaging with \textit{Chandra}. 8/15 galaxy mergers showed dual nuclear X-ray sources, highly suggestive of dual AGNs, with luminosities well in excess of any expected X-ray contributions from star formation. 14/15 mergers showed evidence for high obscuration, as expected from simulations. Interestingly, roughly one third of the nuclei in the sample showed no evidence for AGN activity in the optical. One merger, J0849+1114, exhibited three nuclear X-ray point sources, and follow-up optical and near-IR spectra from the Large Binocular Telescope indicated it is a bona fide triple AGN in a late-stage galaxy merger. Our results indicate that a significant fraction of dual AGNs are missed using optical selection methods and that mid-IR color selection is an effective method for selecting multi-AGN systems. Turning to the well-studied \textit{Swift}-BAT AGN sample, we developed new diagnostic criteria for AGN obscuration using soft X-ray emission and mid-IR colors. This criteria is $>$ 80\% complete for heavily obscured ($10^{24}$ cm$^{-2}$) AGNs, and it will aid in ongoing efforts to identify heavily obscured AGNs in large samples of AGNs, such as those being detected by \textit{eROSITA}.
