NGC6240: A triple nucleus system in the advanced or final state of
  merging
  NGC6240 is a well-studied nearby galaxy system in the process of merging.
Based on optical, X-ray, and radio observations, it is thought to harbor two
active nuclei We carried out a detailed optical 3D spectroscopic study to
investigate the inner region of this system in connection with existing MERLIN
and VLBA data. We observed NGC6240 with very high spatial resolution using the
MUSE instrument in the Narrow-Field Mode with the four-laser GALACSI adaptive
optics system on the ESO VLT under seeing conditions of 0.49 arcsec. Our 3D
spectra cover the wavelength range from 4725 to 9350 AA at a spatial resolution
of ~75 mas. We report the discovery of three nuclei in the final state of
merging within a region of only 1 kpc in the NGC6240 system.Thanks to MUSE we
are able to show that the formerly unresolved southern component actually
consists of two distinct nuclei separated by only 198 pc. In combination with
Gaia data we reach an absolute positional accuracy of only 30 mas that is
essential to compare optical spectra with MERLIN and VLBA radio positions. The
verification and detailed study of a system with three nuclei, two of which are
active and each with a mass in excess of $9\times10^{7} M_{\odot}$, is of great
importance for the understanding of hierarchical galaxy formation via merging
processes since multiple mergers lead to a faster evolution of massive galaxies
in comparison to binary mergers. So far it has been suggested that the
formation of galactic nuclei with multiple supermassive black holes (SMBHs) is
expected to be rare in the local universe.Triple massive black hole systems
might be of fundamental importance for the coalescence of massive black hole
binaries in less than a Hubble time leading to the loudest sources of
gravitational waves in the millihertz regime.
