A newly discovered pair of supermassive black holes (SMBH) are the closest we've ever found to Earth, and to each other. They represent a scientific goldmine, being both easier to investigate than more distant counterparts and revealing a previously unseen stage of the black hole merger process. The finding also indicates we've underestimated how many SMBH's exist in our cosmic neighborhood.
Black holes formed from supernova explosions have been found as close as 1,000 light-years away from us, but supermassive black holes are something quite different. Instead of having masses a few times larger than the Sun, these lie at the heart of galaxies and are measured in millions of solar masses.
The interactions of these gigantic pairs of pure gravitational force can shape the galaxies around them. Unfortunately, most are at such immense distances this is a challenge. Which makes the announcement in Astronomy and Astrophysics of a SMBH pair in the galaxy NGC 7727 significant.
This merging galaxy lies 89 million light-years away. By comparison, the previous closest pair of supermassive black holes, in NGC 6240, are more than five times further from us, although that one has a suspected third partner to spice things up.
Supermassive black holes lose energy as they orbit each other, falling closer and closer together until they merge, creating the most powerful gravitational waves in the universe. The NGC 7727 pair are 1,600 light-years apart, about half the distance of the next closest SMBH pair we have found.
Astronomers identify most of the SMBHs they find through X-rays from gas falling on their accretion disks, but neither of these two is detectible this way, co-author Dr Holger Baumgardt of the University of Queensland told IFLScience. “There's no gas around them,” he said. “It could be temporary, if you came back in a million years they might be passing through a gas cloud and might radiate again.”
Instead, the authors studied the spectra of the stars in each nucleus to see how fast they were moving, calculating the invisible mass required to induce such orbits.
“Our finding implies that there might be many more of these relics of galaxy mergers out there and they may contain many hidden massive black holes that still wait to be found,” said first author Dr Karina Voggel of Observatoire astronomique de Strasbourg in a statement. “It could increase the total number of supermassive black holes known in the local Universe by 30 percent.”
Large galaxies form by either consuming smaller galaxies entirely or stripping away some of their stars in close approaches. However, we mostly deduce this from what such events leave behind. NGC 7277 “is a rare direct glimpse into the process of stripped nuclei formation as well as SMBH assembly through mergers,” the authors conclude.
At 154 million solar masses, the larger SMBH is 24 times as massive as the smaller one. The current mass of the two galaxies is also heavily skewed, but the authors think this is because the larger galaxy has already stripped much of the mass away from the smaller one, and their initial galactic ratio was around five to one.
The time until the two black holes merge could be between a few tens of millions and 200 million years, Baumgardt told IFLScience.