2011ApJ...736L...7L

Hierarchical galaxy mergers will lead to the formation of binary and, in the case of a subsequent merger before a binary coalesces, triple supermassive black holes (SMBHs), given that most massive galaxies harbor SMBHs. A triple of SMBHs becomes visible as a triple active galactic nucleus (AGN) when the BHs accrete large amounts of gas at the same time. Here, we report the discovery of a kiloparsec-scale triple AGN, SDSS J1027+1749 at z = 0.066, from our systematic search for hierarchical mergers of AGNs. The galaxy contains three emission-line nuclei, two of which are offset by 450 and 110 km/s in velocity and by 2.4 and 3.0 kpc in projected separation from the central nucleus. All three nuclei are classified as obscured AGNs based on optical diagnostic emission line ratios, with black hole mass estimates M_• ≳ 10⁸ M_☉ from stellar velocity dispersions measured in the associated stellar components. Based on dynamical friction timescale estimates, the three stellar components in SDSS J1027+1749 will merge in ∼40 Myr, and their associated SMBHs may evolve into a gravitationally interacting triple system in ≲ 200 Myr. Our result sets a lower limit of ∼5x10^–5 for the fraction of kiloparsec-scale triples in optically selected AGNs at z ∼ 0.1.