The host galaxies and narrow-line regions of four double-peaked [OIII] AGNs

Major gas-rich mergers of galaxies are expected to play an important role in triggering and fueling luminous active galactic nuclei (AGNs). The mechanism of AGN fueling during mergers, however, remains poorly understood. We present deep multi-band ($u/r/z$) imaging and long-slit spectroscopy of four double-peaked $\left[ {\rm OIII} \right]$ emitting AGNs. This class of object is likely associated with either kiloparsec-separated binary AGNs or final stage major mergers, although AGNs with complex narrow-line regions (NLRs) are known contaminants. Such objects are of interest since they represent the onset of AGN activity during the merger process. Three of the four double-peaked $\left[ {\rm OIII} \right]$ emitters studied have been confirmed as major mergers using near-infrared imaging and one is a confirmed X-ray binary AGN. All AGNs are luminous, radio-quiet to radio-intermediate, and have redshifts of $0.1\lt z\lt 0.4$. Deep r-band images show that a majority (3/4) of the sources have disturbed host morphologies and tidal features, while the remaining source is morphologically undisturbed down to low surface brightness limits (~27 mag arcsec−2 in r). The lack of morphological disturbances in this galaxy despite the fact that it is a close binary AGN suggests that the merger of a binary black hole can take longer than 1 Gyr. All AGNs hosted by merging galaxies have companions at distances $\leqslant $150 kpc. The NLRs have large sizes (10 kpc $\lt \;r\;\lt $ 100 kpc) and consist of compact clumps with considerable relative velocities between components (~200–650 km s−1). We detect broad, predominantly blue, wings with velocities up to ~1500 km s−1 in $\left[ {\rm OIII} \right]$, indicative of powerful outflows. The outflows are compact ($\lt 5$ kpc) and co-spatial with nuclear regions showing considerable reddening, consistent with enhanced star formation. One source shows an offset between gas and stellar kinematics, consistent with either a bipolar flow or a counter-rotating gas disk. In all other sources, the ionized gas generally follows the stars. We are not able to unambiguously identify the sources as binary AGNs using our data; X-ray or radio data are required for an unambiguous identification. However, the data still yield interesting results for merger triggering of AGNs and timescales of binary black hole mergers.