The existence of binary supermassive black holes (SBHs) is predicted by models of hierarchical galaxy formation. To date, only a single binary SBH has been imaged, at a projected separation of 7.3 pc. Here, we report the detection of a candidate dual SBH with projected separation of 0.35 pc in the gas-rich interacting spiral galaxy NGC 7674 (Mrk 533). This peculiar Seyfert galaxy possesses a roughly 0.7 kpc Z-shaped radio jet. The leading model for the formation of such sources postulates the presence of an uncoalesced binary SBH created during the infall of a satellite galaxy. Using very long baseline interferometry, we imaged the central region of Mrk 533 at radio frequencies of 2, 5, 8 and 15 GHz. Two, possibly inverted-spectrum, radio cores were detected at 15 GHz only. The 8–15 GHz spectral indices of the two cores were ≥−0.33 and ≥−0.38 (±30%), consistent with accreting SBHs. We derived a jet speed of around 0.28c from multi-epoch parsec-scale data of the hotspot region and a source age of ≥ 8.2 × 103 years.
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P.K. and D.V.L. thank D. C. Gabuzda for help in obtaining the 1998 and 2002 VLBA data and provision of expert knowledge on VLBI data reduction. D.M. was supported by the National Science Foundation under grant no. AST 1211602 and the National Aeronautics and Space Administration under grant no. NNX13AG92G. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities.
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Kharb, P., Lal, D.V. & Merritt, D. A candidate sub-parsec binary black hole in the Seyfert galaxy NGC 7674. Nat Astron 1, 727–733 (2017). https://doi.org/10.1038/s41550-017-0256-4