NGC6240: A triple nucleus system in the advanced/final state of merging

NGC6240 is a well studied nearby galaxy system in the process of merging. It has been claimed to harbor two active nuclei based on optical, X-ray and radio observations. We carried out a detailed optical 3D spectroscopic study to investigate the inner region of this system in connection with existing MERLIN/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 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 198 pc only. In combination with Gaia data we reach an absolute positional accuracy of only 30 mas that is essential to compare optical spectra with MERLIN/VLBA radio positions. The verification and detailed study of a system with three nuclei -- two of them being 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 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 mHz regime.