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A candidate sub-parsec binary black hole in the Seyfert galaxy NGC 7674


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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Fig. 1: 15 GHz contour image of Mrk 533 from the EVLA in the A-array configuration showing the C and W hotspots, as well as a roughly 100 mas radio core detected in this galaxy.
Fig. 2: VLBA contour image at 2 GHz showing the southeast (C) and northwest (W) hotspots, along with the position of the cores detected at 15 GHz as a cross.
Fig. 3: VLBI image showing the two cores (C1 and C2) detected at 15 GHz.
Fig. 4: A 2–5 GHz spectral index image of the two hotspots.
Fig. 5: Radio spectrum of the two VLBI cores in Mrk 533, which are only detected at 15 GHz.
Fig. 6: 5 GHz contour images of the southeast hotspot region of Mrk 533 in two different epochs.

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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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P.K. reduced the 2002 VLBA data, coordinated the research and wrote the manuscript. D.V.L. led the VLBA proposals for the 2002 and 1998 data, reduced the 1998 data and provided feedback on the manuscript. D.M. provided the theoretical framework for the project.

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Correspondence to P. Kharb.

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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).

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