Buried AGNs in Advanced Mergers:Mid-infrared color selection as a dual
  AGN finder
  A direct consequence of hierarchical galaxy formation is the existence of
dual supermassive black holes (SMBHs), which may be preferentially triggered as
active galactic nuclei (AGN) during galaxy mergers. Despite decades of
searching, however, dual AGNs are extremely rare, and most have been discovered
serendipitously. Using the all-sky WISE survey, we identified a population of
over 100 morphologically identified interacting galaxies or mergers that
display red mid-infrared colors often associated in extragalactic sources with
powerful AGNs. The vast majority of these advanced mergers are optically
classified as star-forming galaxies suggesting that they may represent an
obscured population of AGNs that cannot be found through optical studies. In
this work, we present Chandra/ACIS observations and near-infrared spectra with
the Large Binocular Telescope of six advanced mergers with projected pair
separations less than ~ 10 kpc. The combined X-ray, near-infrared, and
mid-infrared properties of these mergers provide confirmation that four out of
the six mergers host at least one AGN, with four of the mergers possibly
hosting dual AGNs with projected separations less than ~10 kpc, despite showing
no firm evidence for AGNs based on optical spectroscopic studies. Our results
demonstrate that 1) optical studies miss a significant fraction of single and
dual AGNs in advanced mergers, and 2) mid-infrared pre-selection is extremely
effective in identifying dual AGN candidates in late-stage mergers. Our
multi-wavelength observations suggest that the buried AGNs in these mergers are
highly absorbed, with intrinsic column densities in excess of N_H >10^24cm^-2,
consistent with hydrodynamic simulations.
