Active Galactic Nucleus Pairs from the Sloan Digital Sky Survey. III. Chandra X-Ray Observations Unveil Obscured Double Nuclei

Abstract

We present Chandra ACIS-S X-ray imaging spectroscopy for five dual active galactic nucleus (AGN) candidates. Our targets were drawn from a sample of 1286 [O III ]-selected AGN pairs systematically selected from the Sloan Digital Sky Survey Seventh Data Release. Each of the targets contains two nuclei separated by ∼3–9 kpc in projection, both of which are optically classified as Type 2 (obscured) AGNs based on diagnostic ratios of the narrow emission lines. Combined with independent, empirical star formation rate estimates based on the host-galaxy stellar continua, the new Chandra X-ray observations allow us to evaluate the dual-AGN hypothesis for each merging system. We confirm two (SDSS J0907+5203 and SDSS J1544+0446) of the five targets as bona fide dual AGNs. For the other three targets, the existing data are consistent with the dual-AGN scenario, but we cannot rule out the possibility of stellar/shock heating and/or one AGN ionizing both gaseous components in the merger. The average X-ray-to-[O III ] luminosity ratio in our targets seems to be systematically smaller than that observed in single AGNs but is higher than that seen in dual AGNs selected from AGNs with double-peaked narrow emission lines. We suggest that the systematically smaller X-ray-to-[O III ] luminosity ratio observed in dual AGNs than in single AGNs is due to a high nuclear gas column likely from strong merger-induced inflows. Unlike double-peaked-[O III ]-selected dual AGNs, the new sample selected from resolved galaxy pairs are not subject to the orientation bias caused by the double-peak line-of-sight velocity splitting selection, which also contributes to lowering the X-ray-to-[O III ] luminosity ratio.