SDSS J1056+5516: A Triple AGN or an SMBH Recoil Candidate?

TL;DR

This paper reports the discovery of a kiloparsec-scale triple supermassive black hole system at redshift 0.256, which could be a triple SMBH, a triple AGN, or a recoiling SMBH, providing insights into galaxy evolution and gravitational waves.

Contribution

The discovery of a rare triple SMBH system at kiloparsec scale, with detailed analysis suggesting possible scenarios for its nature, advancing understanding of galaxy mergers and SMBH dynamics.

Findings

Identification of three emission-line nuclei with small velocity offsets

Presence of a 100 kpc tidal arm indicating galaxy merger

Possible scenarios include triple SMBH, triple AGN, or recoiling SMBH

Abstract

We report the discovery of a kiloparsec-scale triple supermassive black hole system at $z=0.256$: SDSS J1056+5516, discovered by our systematic search for binary quasars. The system contains three strong emission-line nuclei, which are offset by $<250~{\rm km~s^{-1}}$ and by 15-18 kpc in projected separation, suggesting that the nuclei belong to the same physical structure. The system includes a tidal arm feature spanning $\sim100$ kpc in projected distance at the systems' redshift, inhabiting an ongoing or recent galaxy merger. Based on our results, such a structure can only satisfy one of the three scenarios; a triple supermasive black hole (SMBH) interacting system, a triple AGN, or a recoiling SMBH. Each of these scenarios is unique for our understanding of the hierarchical growth of galaxies, AGN triggering, and gravitational waves.