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


Abstract in English

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 parsecs. 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 $sim$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 (VLBI), 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 are $ge-0.33$ and $ge-0.38$ ($pm 30%$), consistent with accreting SBHs. We derive a jet speed $sim0.28c$ from multi-epoch parsec-scale data of the hotspot region, and a source age $ge8.2times10^3$ yrs.

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