Centre de Recherche Astrophysique de Lyon

Text adapted from Göttingen and Potsdam press-release universities.
21st Novembre 2019.

An international research team including Roland Bacon from CRAL proved for the first time that the galaxy NGC 6240 contains three supermassive black holes, using the MUSE instrument at the Very Large Telescope of ESO. The unique observations, published in the journal Astronomy & Astrophysics, show them close to each other in the core of the galaxy. The study points to simultaneous merging processes during the formation of the largest galaxies in the universe.

Galaxies typically consist of 100 to 300 billion stars and host a black hole with a mass of several million up to 100 million solar masses at their centers. The galaxy known as NGC 6240 is termed an irregular galaxy due to its particular shape. Until now, astronomers have assumed that it was formed by the collision of two smaller galaxies and therefore contains two black holes in its core. These galactic ancestors moved towards each other at velocities of several 100 km/s and are still in the process of merging. The galaxy system at a distance of approximately 300 million light years – close by cosmic standards –, has been studied in detail at all wavelengths, and has so far been regarded as a prototype for the interaction of galaxies.

Through our observations with extremely high spatial resolution the team was able to show that the interacting galaxy system NGC 6240 hosts not two – as previously assumed – but three supermassive black holes in its center. Each of the three heavyweights has a mass of more than 90 million Suns. They are located in a region of space of less than 3000 light-years across, i.e. in less than one hundredth of the total size of the galaxy. Such a concentration of three supermassive black holes has so far never been discovered in the universe. The present case provides evidence of a simultaneous merging process of three galaxies along with their central black holes.

The discovery of this triple system is of fundamental importance for understanding the evolution of galaxies over time. So far it has not been possible to explain how the largest and most massive galaxies, which we know from our cosmic environment in the "present time", were formed merely through normal galaxy interaction and merging processes over the course of the last approximately 14 billion years, the age of our universe. If, however, simultaneous merging processes of several galaxies took place, then the largest galaxies with their central supermassive black holes were able to evolve much faster. These observations provide the first indication of this scenario.

For the unique high-precision observations of NGC 6240 with the 8m VLT telescope in Chile, the 3D MUSE spectrograph was used in spatial high-resolution mode together with four artificially generated laser stars and an adaptive optics system. Thanks to the sophisticated technology, images are obtained with a sharpness similar to that of the Hubble Space Telescope but additionally contain a spectrum for each image pixel. These spectra were decisive in determining the motion and masses of the supermassive black holes in NGC 6240.

The astrophysics assume that the observed, imminent merging of the supermassive black holes in a few million years will also generate very strong gravitational waves. In the foreseeable future, signals of similar objects can be measured with the planned satellite-based gravitational wave detector LISA and further merging systems can be discovered.

The irregular galaxy NGC 6240. New observations show that it harbours not two but three supermassive black holes at the core. The northern black hole (N) is active and was known before. The zoomed-in new high-spatial resolution image shows that the southern component consists of two supermassive black holes (S1 and S2). The green colour indicates the distribution of gas ionized by radiation surrounding the black holes. The red lines show the contours of the star light from the galaxy and the length of the white bar corresponds to 1000 light years. Credit: P. Weilbacher (AIP), NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

Publication:
W. Kollatschny (1), P. M. Weilbacher (2), M. W. Ochmann (1), D. Chelouche (3), A. Monreal-Ibero (4,5) R. Bacon (6), T. Contini (7)
NGC6240: A triple nucleus system in the advanced or final state of merging, Astronomy & Astrophysics, 2019

(1) Institut für Astrophysik, Universität Göttingen, Germany
(2) Leibniz-Institut für Astrophysik Potsdam, Germany
(3) Physics Department and the Haifa Research Center for Theoretical Physics and Astrophysics, University of Haifa, Israel
(4, 5) Instituto de Astrofísica de Canarias & Universidad de La Laguna, Dpto. Astrofísica, Spain
(6) Univ Lyon, Univ Lyon1, Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, France
(7) Institut de Recherche en Astrophysique et Planétologie , Université de Toulouse, CNRS, UPS, France

French Scientific contact:
Dr. Roland Bacon @CRAL UMR5574 (UCBL1, ENSL, CNRS)