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The existence of binary supermassive black holes (SBHs) is predicted bymodels of hierarchical galaxy formation. To date, only a single binary SBH hasbeen imaged, at a projected separation of 7.3 parsecs. Here we report thedetection of a candidate dual SBH with projected separation of 0.35 pc in thegas-rich interacting spiral galaxy NGC 7674 (Mrk 533). This peculiar Seyfertgalaxy possesses a $\sim$0.7 kpc Z-shaped radio jet; the leading model for theformation of such sources postulates the presence of an uncoalesced binary SBHcreated during the infall of a satellite galaxy. Using very long baselineinterferometry (VLBI), we imaged the central region of Mrk 533 at radiofrequencies of 2, 5, 8 and 15 GHz. Two, possibly inverted-spectrum radio coreswere detected at 15 GHz only; the 8-15 GHz spectral indices of the two coresare $\ge-0.33$ and $\ge-0.38$ ($\pm 30\%$), consistent with accreting SBHs. Wederive a jet speed $\sim0.28c$ from multi-epoch parsec-scale data of thehotspot region, and a source age $\ge8.2\times10^3$ yrs.

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