Kiloparsec-Scale Spatial Offsets In Double-Peaked Narrow-Line Active Galactic Nuclei. I. Markers For Selection Of Compelling Dual Active Galactic Nucleus Candidates
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Merger-remnant galaxies with kiloparsec (kpc) scale separation dual active galactic nuclei (AGNs) should be widespread as a consequence of galaxy mergers and triggered gas accretion onto supermassive black holes, yet very few dual AGNs have been observed. Galaxies with double-peaked narrow AGN emission lines in the Sloan Digital Sky Survey (SDSS) are plausible dual AGN candidates, but their double-peaked profiles could also be the result of gas kinematics or AGN-driven outflows and jets on small or large scales. To help distinguish between these scenarios, we have obtained spatial profiles of the AGN emission via follow-up long-slit spectroscopy of 81 double-peaked narrow-line AGNs in SDSS at 0.03 <= z <= 0.36 using Lick, Palomar, and MMT Observatories. We find that all 81 systems exhibit double AGN emission components with similar to kpc projected spatial separations on the sky (0.2 h(70)(-1) kpc < Delta x < 5.5 h(70)(-1) kpc; median Delta x = 1.1 h(70)(-1) kpc), which suggests that they are produced by kiloparsec-scale dual AGNs or kiloparsec-scale outflows, jets, or rotating gaseous disks. Further, the objects split into two subpopulations based on the spatial extent of the double emission components and the correlation between projected spatial separations and line-of-sight velocity separations. These results suggest that the subsample (58(-6)(+5)%) of the objects with spatially compact emission components may be preferentially produced by dual AGNs, while the subsample (42(-5)(+6)%) with spatially extended emission components may be preferentially produced by AGN outflows. We also find that for 32(-6)(+8)% of the sample the two AGN emission components are preferentially aligned with the host galaxy major axis, as expected for dual AGNs orbiting in the host galaxy potential. Our results both narrow the list of possible physical mechanisms producing the double AGN components, and suggest several observational criteria for selecting the most promising dual AGN candidates from the full sample of double-peaked narrow-line AGNs. Using these criteria, we determine the 17 most compelling dual AGN candidates in our sample.