Mergers in double-peaked [O III] active galactic nuclei

Abstract

As a natural consequence of galaxy mergers, binary active galactic nuclei (AGNs) should be commonplace. Nevertheless, observational confirmations are rare, especially for binaries with separations less than 10kpc. Such a system may show two sets of narrow emission lines in a single spectrum owing to the orbital motion of the binary. We have obtained high-resolution near-infrared images of 50 double-peaked [O III]λ5007 AGNs with the KeckII laser guide star adaptive optics system. The Sloan Digital Sky Survey sample is compiled from the literature and consists of 17 type-1 AGNs between 0.18 < z < 0.56 and 33 type-2 AGNs between 0.03 < z < 0.24. The new images reveal eight type-1 and eight type-2 sources that are apparently undergoing mergers. These are strong candidates ofkpc-scale binary AGNs because they show multiple components separated between 0.6 and 12kpc and often disturbed morphologies. Because most of the type-1s are at higher redshifts than the type-2s, the higher merger fraction of type-1s (47% 20%) compared to that of type-2s (24% 10%) can be attributed to the general evolution of galaxy merger fraction with redshift. Furthermore, we show that AGN mergers are outliers of the MBH-σ relation because of overestimated stellar velocity dispersions, illustrating the importance of removing mergers from the samples defining the M BH-σ relations. Finally, we find that the emission-line properties are indistinguishable for spatially resolved and unresolved sources, emphasizing that scenarios involving a single AGN can produce the same double-peaked line profiles and they account for at least 70% of the double-peaked [O III] AGNs. © 2011. The American Astronomical Society. All rights reserved.