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

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 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 ∼100 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.