Three supermassive black holes discovered at the core of one galaxy

Hubble image of irregular Galaxy NGC 6240 with a zoom-in on its central region, which shows MUSE-data of three black holes.

The irregular galaxy NGC 6240. New observations show that it harbours not two but three supermassive black holes at its core. The northern black hole (N) is active and was previously known. 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 surrounding the black holes that is ionized by radiation. 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)
Nov. 21, 2019 //

An international research team led by scientists from Göttingen and Potsdam have for the first time shown that the galaxy NGC 6240 contains three supermassive black holes. The unique observations, published in the journal Astronomy & Astrophysics, reveal the black holes 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.

Massive galaxies like the Milky Way typically consist of hundreds of billions of stars and host a black hole with a mass of several million up to several 100 million solar masses at their centres. The galaxy known as NGC 6240 is classified as 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 is around 300 million light years away from us – 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 we were able to show that the interacting galaxy system NGC 6240 hosts not two – as previously assumed – but three supermassive black holes in its centre,” reports Professor Wolfram Kollatschny from the University of Göttingen, the leading author of the study. Each of the three heavyweights has a mass of more than 90 million Suns. They are located in a volume 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,” adds Dr. Peter Weilbacher of the Leibniz Institute for Astrophysics Potsdam (AIP). “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 14 billion years, i.e. the approximate 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,” Peter Weilbacher summarizes. “Our observations provide the first indication of this scenario.”

The unique high-precision observations of galaxy NGC 6240 were obtained using the 3D MUSE spectrograph mounted on the 8m VLT telescope in Chile, a telescope operated by the European Southern Observatory (ESO). The spectrograph was used in spatial high-resolution mode together with four artificially generated laser stars and an adaptive optics system. Thanks to this sophisticated technology, images could be obtained with a sharpness similar to that of the Hubble Space Telescope, but with the added benefit of having a spectrum for each image pixel. These spectra were essential in determining the motion and masses of the supermassive black holes in NGC 6240.

The scientists predict that the 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.

Further information

Original Publication

W. Kollatschny, P. M. Weilbacher, M. W. Ochmann, D. Chelouche, A. Monreal-Ibero; R. Bacon, T. Contini: NGC6240: A triple nucleus system in the advanced or final state of merging, Astronomy & Astrophysics, 2019


University of Göttingen press release

The key areas of research at the Leibniz Institute for Astrophysics Potsdam (AIP) are cosmic magnetic fields and extragalactic astrophysics. A considerable part of the institute's efforts aims at the development of research technology in the fields of spectroscopy, robotic telescopes, and E-science. The AIP is the successor of the Berlin Observatory founded in 1700 and of the Astrophysical Observatory of Potsdam founded in 1874. The latter was the world's first observatory to emphasize explicitly the research area of astrophysics. The AIP has been a member of the Leibniz Association since 1992.
Last update: 30. March 2021