Mergers in Double-peaked [O III] Active Galactic Nuclei

Creators: Fu, Hai; Myers, Adam D.; Djorgovski, S. G.; Yan, Lin

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 10 kpc. 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 Keck II 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 of kpc-scale binary AGNs because they show multiple components separated between 0.6 and 12 kpc 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 M_(BH)^-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.

Additional Information

© 2011 The American Astronomical Society. Received 2011 March 21; accepted 2011 March 22; published 2011 May 11. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We thank the referee for a careful reading of the manuscript and for comments that helped to improve the paper. A.D.M. acknowledges partial support from NASA (grants NNX08AJ28G and GO9-0114). S.G.D. was partially supported by NSF grant AST-0909182 and the Ajax Foundation. The authors recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Facilities: Keck:II (LGSAO/NIRC2, LGSAO/OSIRIS), Sloan, FLWO:2MASS, CTIO:2MASS

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