A Trio of Massive Black Holes Caught in the Act of Merging
  We report the discovery of SDSS J0849+1114 as the first known triple Type 2
Seyfert nucleus. It represents three active black holes that are identified
from new spatially resolved optical slit spectroscopy using the Dual Imaging
Spectrograph on the 3.5 m telescope at the Apache Point Observatory. We also
present new complementary observations including the Hubble Space Telescope
Wide Field Camera 3 U- and Y-band imaging, Chandra Advanced CCD Imaging
Spectrometer S-array X-ray 0.5--8 keV imaging spectroscopy, and NSF Karl G.
Jansky Very Large Array radio 9.0 GHz imaging in its most extended A
configuration. These comprehensive multiwavelength observations, when combined
together, strongly suggest that all three nuclei are active galactic nuclei.
While they are now still at kiloparsec-scale separations, where the host-galaxy
gravitational potential dominates, the black holes may evolve into a bound
triple system in $\lesssim$2 Gyr. These triple merger systems may explain the
overly massive stellar cores that have been observed in some elliptical
galaxies such as M87, which are expected to be unique gravitational wave
sources. Similar systems may be more common in the early universe, when galaxy
mergers are thought to have been more frequent.
