Three Supermassive Black Holes Discovered at the Heart of One Galaxy

Astronomers have discovered three supermassive black holes at the heart of a single galaxy.

According to a study published in the journal Astronomy & Astrophysics, the black holes lie close to each other at the core of the galaxy known as NGC 6240—which is located around 300 million light-years away from us. This finding, the researchers say, casts new light on how the largest galaxies in the universe form.

Scientists think that most large galaxies in the universe—which have hundreds of billions of stars—host a supermassive black hole at their center. These black holes are believed to have masses hundreds of millions of times greater than that of the sun.

NGC 6240 is slightly unusual because of its irregular shape. Scientists long thought that it was formed by the collision of two smaller galaxies and, thus, contained two black holes at its center. However, the latest high-resolution observations revealed something unexpected.

"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," Wolfram Kollatschny, lead author of the study from the University of Göttingen, Germany, said in a statement.

The astronomers say that each of these supermassive black holes has a mass more than 90 million times greater than that of the sun. They are located in a region of space that is less than 3,000 light-years across.

"Up until now, such a concentration of three supermassive black holes had never been discovered in the universe," Peter Weilbacher, another of the study from the Leibniz Institute for Astrophysics Potsdam, said in a statement. "The present case provides evidence of a simultaneous merging process of three galaxies along with their central black holes."

 NGC 6240, supermassive black holes
The irregular galaxy NGC 6240. New observations show that it harbors not two but three supermassive black holes at its 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 color indicates the distribution of gas ionized by radiation surrounding the black holes. The red lines show the contours of the starlight from the galaxy and the length of the white bar corresponds to 1,000 light-years. P Weilbacher AIP), NASA, ESA, the Hubble Heritage STScI/AURA-ESA/Hubble Collaboration, and A Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University

This is significant because until now, astronomers have not been able to explain how the largest and most massive galaxies formed by normal interaction and merging processes over the age of the universe—approximately 14 billion years.

"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 said. "Our observations provide the first indication of this scenario."

The supermassive black holes were identified using the 8-meter Very Large Telescope (VLT) in Chile—which is operated by the European Southern Observatory. The VLT is the world's most advanced visible-light astronomical observatory located in Chile's Atacama Desert.

The VLT array consists of four Unit Telescopes with main mirrors measuring 8.2 meters in diameter. These can work together by combining the light beams they detect using a complex system of subterranean mirrors. This enables astronomers to reconstruct images with a resolution that is equivalent to distinguishing the two headlights of a car at the distance of the Moon.