The Nature of Double-Peaked [O III] Active Galactic Nuclei

Creators: Fu, Hai; Myers, Adam D.; Stockton, Alan; Djorgovski, S. G.; Aldering, G.; Rich, Jeffrey A.

Abstract

Active galactic nuclei (AGNs) with double-peaked [O III] lines are suspected to be sub-kpc or kpc-scale binary AGNs. However, pure gas kinematics can produce the same double-peaked line profile in spatially integrated spectra. Here we combine integral-field spectroscopy and high-resolution imaging of 42 double-peaked [O III] AGNs from the Sloan Digital Sky Survey to investigate the constituents of the population. We find two binary AGNs where the line splitting is driven by the orbital motion of the merging nuclei. Such objects account for only ~2% of the double-peaked AGNs. Almost all (~98%) of the double-peaked AGNs were selected because of gas kinematics; and half of those show spatially resolved narrow-line regions that extend 4-20 kpc from the nuclei. Serendipitously, we find two spectrally unresolved binary AGNs where gas kinematics produced the double-peaked [O III] lines. The relatively frequent serendipitous discoveries indicate that only ~1% of binary AGNs would appear double-peaked in Sloan spectra and 2.2^(+2.5)_(–0.8)% of all Sloan AGNs are binary AGNs. Therefore, the double-peaked sample does not offer much advantage over any other AGN samples in finding binary AGNs. The binary AGN fraction implies an elevated AGN duty cycle (8^(+8)_(–3)%), suggesting galaxy interactions enhance nuclear accretion. We illustrate that integral-field spectroscopy is crucial for identifying binary AGNs: several objects previously classified as "binary AGNs" with long-slit spectra are most likely single AGNs with extended narrow-line regions (ENLRs). The formation of ENLRs driven by radiation pressure is also discussed.

Additional Information

© 2012 American Astronomical Society. Received 2011 July 15; accepted 2011 October 24; published 2011 December 29. 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. The authors thank Steven Rodney, Nick Moskovitz, Mark Willman, and Eric Gaidos for advice on SNIFS observations, and Jessica Lu, Tucker Jones, Peter Capak, and Nick Scoville for helpful discussions. We thank the referee for cogent comments that helped improve the paper. A.D.M. is a research fellow of the Alexander von Humboldt Foundation of Germany. A.S. was partially supported by NSF grant AST-0807900. S.G.D. was partially supported by NSF grant AST-0909182 and the Ajax Foundation. G.A. was supported by the Director, Office of Science, Office of High Energy Physics, of the US Department of Energy under Contract No. DE-AC02-05CH11231. The authors wish to 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. Facilities: UH:2.2m (SNIFS), Keck:II (LGSAO/NIRC2, LGSAO/OSIRIS), Sloan

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