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

TL;DR

This paper reports the detection of a candidate dual supermassive black hole with a separation of 0.35 parsecs in NGC 7674, using VLBI radio observations, suggesting a possible binary black hole system in a galaxy with a Z-shaped jet.

Contribution

First detection of a candidate sub-parsec binary supermassive black hole in NGC 7674 using multi-frequency VLBI imaging.

Findings

Detected two radio cores at 15 GHz with spectral indices consistent with accreting SBHs.

Derived jet speed of approximately 0.28c from multi-epoch data.

Estimated source age of at least 8,200 years.

Abstract

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 $\ge8.2\times10^3$ yrs.