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 ∼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 ≥−0.33 and ≥−0.38 (±30%), consistent with accreting SBHs. We derive a jet speed ∼0.28c from multi-epoch parsec-scale data of the hotspot region, and a source age ≥8.2×103 yrs.
@article{arxiv.1709.06258,
title = {A candidate sub-parsec binary black hole in the Seyfert galaxy NGC 7674},
author = {Preeti Kharb and Dharam Vir Lal and David Merritt},
journal= {arXiv preprint arXiv:1709.06258},
year = {2017}
}