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

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 $<250\mathrm{k}\mathrm{m}{\mathrm{s}}^{-1}$ 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 $\sim 100$ 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.