Presentation #213.02D in the session AGN and Quasars III.
Galaxy collisions, a ubiquitous phenomenon in the Universe, are predicted to be a critical avenue for galaxy and black hole growth and evolution. During a merger event, gravitational torques drive reservoirs of gas and dust toward the galactic cores, and these inflows are consequently accreted by the central supermassive black holes, which then manifest as active galactic nuclei (AGNs). Dual AGNs are expected to occur in late-stage mergers, where the black holes are predicted to experience their most rapid period of growth. In our Chandra investigation of 15 late-stage mergers preselected with WISE, we found dual AGNs or candidate duals in 8 out of 15 mergers, many of which show no evidence for AGNs in the optical. Our multiwavelength observations suggest that the AGNs in these mergers are highly absorbed, with intrinsic column densities in excess of NH > 1023-1024 cm-2, consistent with hydrodynamic simulations. One of these mergers, SDSS J0849+1114, was in fact a triple galaxy merger, and exhibited three nuclear X-ray sources detected by Chandra. Through a multiwavelength follow-up program, we demonstrated that SDSS J0849+1114 represents the most compelling case for a triple AGN in the literature and has since been confirmed by two further studies. We will also discuss more recent work related to obscured AGN growth more generally, highlighting a new X-ray/mid-IR diagnostic for AGN obscuration identified in our study of Swift/BAT AGNs. This diagnostic relies upon the well-known LX, Obs./L12 μm luminosity ratio as well as mid-IR colors to select heavily obscured Swift/BAT AGNs ( log[NH] > 23.5 ) with high completeness and reliability. Our new obscuration diagnostic could be used to differentiate between unobscured and heavily obscured AGNs in future, large samples of AGNs, such as those now being detected by the eROSITA all-sky survey.