Research team discovers
three supermassive
black holes at the core of one galaxy
'University of Göttingen – November 21,
2019 –- An international research team led by scientists from Göttingen and
Potsdam proved for the first time that the galaxy NGC 6240 contains three
supermassive black holes. The unique observations, published in the journal
Astronomy & Astrophysics, 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.
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 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.
"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 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 3000 light-years across, i.e. in
less than one hundredth of the total size of the galaxy. "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."
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, ie 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," Peter Weilbacher summarizes. "Our observations provide the
first indication of this scenario."
For the unique high-precision
observations of the galaxy NGC 6240 using the 8 metre VLT, a telescope operated
by the European Southern Observatory 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 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.
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