Astronomers have discovered the closest pair of supermassive black holes to Earth. Not only are they the most proximate objects of this type to our planet, but they are also closer to each other than any such pairing ever before observed.
This closeness indicates that the black holes, which are 89 million light-years away from Earth in the galaxy NGC 7727 in the constellation Aquarius, are moving towards each other and will eventually collide and merge, forming an even more massive black hole.
The previous closest supermassive black hole pairing is located 470 million light-years from our planet.
The two supermassive black holes are separated by just 1,600 light-years, half the separation of the previous record holders for the smallest separation, astronomer at the Strasbourg Observatory in France, Karina Voggel, said.
Voggel, who is the lead author of a paper documenting the team's findings, told Newsweek: "This system has the two closest supermassive black holes ever discovered. One of these giant black holes is at the center of the galaxy where we expect them typically.
"The second black hole is not in the center but a little bit offset from it. We have never found a supermassive black hole pair at such a small distance to each other."
Voggel goes on to explain what makes this pairing even more extraordinary is that the supermassive black holes involved are in close proximity because of a previous huge merger, this time between galaxies.
She said: "The reason that there are two black holes here is that another galaxy merged with the main galaxy a while ago, and that also had a central black hole."
Astrophysicists and astronomers believe that most galaxies have a supermassive black hole at their centers. Our galaxy, for instance, has the supermassive black hole Sagittarius A* (Sgr A*) at its heart.
Voggel and her team were able to determine the masses of the two black holes by looking at how the gravitational pull of the black holes influences the way the stars around them move.
They discovered that the bigger black hole, located right at the core of NGC 7727, was found to have a mass almost 154 million times that of the sun, while its companion is 6.3 million times the mass of our stars. That means that even the smaller supermassive black hole in this merging pair is larger than Sgr A*, which has a mass around four million times that of the sun.
Voggel explained to Newsweek that this finding is important because even though these black holes won't collide and merge for at least 250 million years, an immensely long time for us, but a blink of an eye in a universe over 13 billion years old, it gives us the chance to study such a system before it merges.
This could solve a long-standing puzzle in astrophysics of how supermassive black holes grow to such tremendous sizes.
"We do not yet understand how black holes grow their mass. One of the ways could be through the merging with supermassive black holes from other galaxies," Voggel said.
"However, this process is very hard to observe in itself as most galaxies are much further away than this, and when they are far away one cannot measure the exact masses of these black holes.
"This is the most detailed glimpse into such a system before it will merge into one big black hole."
Sebastian Kamann is an astrophysicist at Liverpool John Moores University who was not involved in the research.
He told Newsweek: "What's exciting about this discovery is that it tells us something about how galaxies grow.
"The 'less massive' supermassive black hole is the last relic of a galaxy that has already been accreted by its more massive neighbor. In the distant future—about one billion years according to the paper—its surviving SMBH will also merge with its even more massive counterpart."
Astronomers have studied the galaxy NGC 7727 before but had been unable to determine if the galaxy actually hosted the two black holes as the large amounts of high-energy radiation coming from their immediate surroundings (which would otherwise give them away) was absent.
Voggel and her team were able to confirm the presence of the supermassive black hole pair by using the Multi-Unit Spectroscopic Explorer (MUSE) on ESO's Very Large Telescope (VLT).
The observations with MUSE, an instrument Voggel learned to work with during her time as a student at ESO, were followed up with data collected by additional data from NASA's Hubble Space Telescope.
Co-author of the paper, astronomer at ESO in Chile, and Head of ESO Paranal Science Operations, Steffen Mieske, said that further investigations of the black holes will involve the ESO's Extremely Large Telescope (ELT), currently under construction in Northern Chile's Atacama Desert.
Kamann believes that follow-up investigations of similar merging supermassive black holes could also rely on an entirely different type of astronomy, the measurement of gravitational waves.
He added: "The Laser Interferometer Space Antenna (LISA) satellite mission by ESA, scheduled for launch during the next decade, will scan the universe for such mergers of massive black holes.
"The paper gives us a first glimpse of what LISA will observe."
