Searching for Dual AGNs in Galaxy Mergers: Understanding Double-Peaked [O III] as a Selection Method
When galaxies merge, gas may accrete onto both central supermassive black holes. Thus, one expects to see close pairs of active galactic nuclei (AGNs), or dual AGNs, in a fraction of galaxy mergers. The existence and statistics of dual AGNs provide an important probe into hierarchical galaxy formation models, accretion-triggering mechanisms, galaxy merger rates, and black hole growth. However, finding them remains a challenge. The presence of double-peaked [O III] has been proposed as a technique to select dual AGNs efficiently. We studied a sample of double-peaked narrow [O III] emitting AGNs from the Sloan Digital Sky Survey. By obtaining new and archival high spatial resolution images taken with the Keck II Laser Guide Star Adaptive Optics system and the near-infrared (IR) camera NIRC2, we showed that 31%+/-3% of double-peaked [O III] emission line SDSS AGNs have two spatial components within a 3" radius.
However, the imaging discussed above does not show whether or how the double emission line structure relates to the presence of a companion. Spatially resolved spectroscopy or X-ray observations can confirm these galaxy pairs as systems containing two AGNs. We followed up these spatially-double candidate dual AGNs with integral field spectroscopy from Keck OSIRIS and Gemini GMOS and with long-slit spectroscopy from Keck NIRSPEC and Shane Kast Double Spectrograph. We find double-peaked lines are sometimes dual AGN and sometimes outflows or narrow line kinematics. We also performed Chandra X-ray ACIS-S observations on 12 double-peaked candidate dual AGNs. Using our observations and 8 archival observations, we compare the distribution of X-ray photons to our spatially double near-IR images, measure X-ray luminosities and hardness ratios, and estimate column densities. By assessing what fraction of double-peaked emission line SDSS AGNs are true dual AGNs, we can better determine whether double-peaked [O III] is an efficient dual AGN indicator and constrain the statistics of dual AGNs.