Cosmic Train Wreck by Massive Black Holes: Discovery of a kpc-Scale
  Triple Active Galactic Nucleus
  Hierarchical galaxy mergers will lead to the formation of binary and, in the
case of a subsequent merger before a binary coalesce, 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
kpc-scale triple AGN, SDSSJ1027+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_BH ~> 10^8 M_sun from stellar velocity
dispersions measured in the associated stellar components. Based on dynamical
friction timescale estimates, the three stellar components in SDSSJ1027+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 ~ 5 x 10^(-5) for the fraction of kpc-scale triples in optically
selected AGNs at z ~ 0.1.
