Massive galaxies like our Milky Way and Andromeda typically consist of hundreds of billions of stars and host a single, central black hole with a mass of several million up to several 100 million solar masses at their centers. In 2019, an international research team led by scientists from Göttingen and Potsdam proved for the first time that the irregular galaxy NGC 6240, the Starfish Galaxy, contains three supermassive black holes, the remnant of mergers between three smaller galaxies.
The observations (above) show 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.
The galaxy known as NGC 6240 is known 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, which 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.
Three, not Two, Supermassive Objects at Its Core
“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 center,” reports Professor Wolfram Kollatschny from the University of Göttingen, the lead author of the study. Each of the three heavyweights has a mass of more than 90 million Suns. They are located in a region of space less than 3,000 light-years across, i.e. in less than one hundredth of the total size of the galaxy.
Serendipity at Work
We asked Wolfram Kollatschny for his reaction when he discovered that there were three, not two, supermassive black holes (SMBHs) at the galaxy’s center. He replied to The Daily Galaxy in an email: “I stumbled upon the double southern nucleus by accident, and was really surprised to find two nuclei in such close proximity to each other. When I began to inspect the MUSE 3D spectra I saw two southern emission regions in addition to the known northern nucleus. The two southern sources were not resolved in a MUSE 3D spectrum that has been taken three years before. Only the improvement in the spatial resolution – based on a new adaptive optics system with four laser stars – made it possible to resolve the two southern sources. I analyzed the velocity field of the gas and of the stellar component in the next step. That was an important test to prove that the observed two southern sources were real nuclei.”
One of Its Kind in the Universe?
“I do not know of any subsequent discoveries of other three SMBH systems in such a close environment,” he concluded in his email.
“Up until now, such a concentration of three supermassive black holes had 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.”
Importance for Understanding the Evolution of Galaxies
The discovery of this triple system is of fundamental importance for understanding the evolution of galaxies over time. Until now 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 just by normal galaxy interaction and merging processes over the course of the previous 14 billion years approximately, i.e., 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,” Weilbacher summarizes. “Our observations provide the first indication of this scenario.”
For the unique high-precision observations of the galaxy NGC 6240, they utilized the 8-meter Very Large Telescope, a telescope operated by the European Southern Observatory in Chile, combined with the 3D MUSE spectrograph, operated 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.
The Last Word
We asked Kollatschny whether they were able to estimate what the size of the object will be at the “end” of the merging process. He replied, “The two southern black holes have masses of 7.1 x 10^8 M_solar and 0.9 x 10^8 M_solar. The final black hole will have a mass of 8 x 10^8 M_solar after the merging process. The radius of the black hole is directly proportional to the mass (the so called Schwarzschild radius). The third (northern) black hole is important for speeding up the merging process of the two southern black holes.”
The merging process of two nuclei consists of three phases, Kollatschny told The Daily Galaxy: dynamical friction, stellar hardening, and gravitational wave emission.
“I do not know of any subsequent discoveries of other three SMBH systems in such a close environment,” Kollatschny concluded.
The scientists 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.
Original Publication: Wolfram Kollatschny et al. NGC6240: A triple nucleus system in the advanced or final state of merging. Astronomy & Astrophysics 2019. Doi: 10.1051/0004-6361/201936540.
Image credit: P Weilbacher (AIP), NASA, ESA, The Hubble Heritage (STSCI/AURA)-Esa Hubble Collaboration, and A Evans (University of Virginia/NRAO/Stony Brook University.