In new finding astronomers discovered galactic merger of three supermassive black holes. NGC 6240 has always stood out for its peculiar shape and unusual infrared brightness among the myriad galaxies we’ve seen in the night sky. It was believed to be the result of the colliding of two galaxies-so far.
Astronomers announced evidence of a double active nucleus, two active supermassive black holes in the middle of NGC 6240, all the way back in 1983. Today, for the first time, a third supermassive black hole has been discovered by researchers.
This new finding suggests that the process of merging is not two, but three galaxies, each bringing its own supermassive black hole galactic nucleus to the party.
“Through our observations with extremely high spatial resolution,” explained astrophysicist Wolfram Kollatschny of University of Göttingen in Germany, “we have been able to show that the interacting galaxy system NGC 6240 hosts not two-as previously assumed-but three supermassive black holes in its center.”
Numerous galaxy observations have been carried out since 1983, supporting the presence of two active galactic nuclei at the core of NGC 6240, which is 300 million light-years away as the photon flies.
Because the supermassive black holes are so close together, however, it has remained elusive exactly where they are within the bright center.
For high-resolution spectroscopic measurements of the universe, Kollatschny and his team used the MUSE 3D spectrograph mounted on the eight-meter Very Large Telescope of the European Southern Observatory in Chile.
Three nodes in the center of NGC 6240 were revealed in this images-one northern component and two southern. The new finding does not indicate, however, that the previous research is wrong; the new evidence indicates that only two of the black holes actively accreting matter, and the third is dormant.
Each of the supermassive black holes is more than 90 million times the Sun’s mass (the supermassive black hole of the Milky Way, Sagittarius A*, is 4 million solar masses). The three are locked in an orbit in an area of less than 1 kiloparsec (3,260 light-years), spiralling slowly inward. And a gap of only 198 parsecs (645 light-years) separates the two southern black holes.
“Such a concentration of three supermassive black holes has so far never been found in the Universe,” said astrophysicist Peter Weilbacher of the Leibniz Institute for Astrophysics Potsdam in Germany.
“The present case provides evidence of three galaxies simultaneously merging together with their central black holes.”
Another triple merger was discovered earlier this year, with three supermassive black holes being formed at the core of a galaxy called SDSS J084905.51+11447.2; but that system had separations of about 10 kiloparsec between each pair of black holes.
That the black holes at the core of NGC 6240 are similar to each other means that NGC 6240 is at a later stage of its fusion, a cycle that takes about a billion years. This advanced stage also means that the galaxy is closer to what is known as the final parsec problem.
As we reported earlier, the black holes of two merging galaxies are inexorably drawn together according to the theoretical models, passing their orbital energy to the gas and stars around them, and thus orbiting in an ever-tighter spiral.
We know that black holes will eventually come together in pairs of stellar mass and form a single object. There is a theoretical problem with supermassive black holes.
The area of space to which they can transfer energy often shrinks as their orbit shrinks. By the time they are one parsec apart (around 3.2 light-years), this region of space is no longer technically large enough to support further orbital decay, and they remain in a stable binary orbit-potentially for billions of years. This balance is known as ‘final parsec problem’.
Triple mergers could be a solution to this problem, as the third black hole could provide the extra kick that objects need to close the final gap.
Of course, the black holes at the center of NGC 6240 will not soon get anywhere near that final parsec-it could take another billion years or two, and who even knows if humanity will be there at that point.
But it is expected that the black holes will produce gravitational waves. We can’t detect them yet, but we can figure out how to detect them with future instruments by studying systems like these and figure out what’s going on at that last parsec.
The research was published in Astronomy & Astrophysics.