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 can not 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 \sim 10 Â 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 \sim 10 Â 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 \sim N _ { \mathrm { H } } > 10 ^ { 24 } Â cm ^ { -2 } , consistent with hydrodynamic simulations .