This image shows close-up (left) and wide-field (right) views of the two bright galactic nuclei that each host a supermassive black hole. Credit: ESO

Using the Very Large Telescope of the European Southern Observatory (ESO), astronomers revealed the presence of the closest pair of supermassive black holes to Earth ever observed, which will eventually merge into a giant black hole.

Located in the galaxy NGC 7727, in the constellation Aquarius, the pair of supermassive black holes is about 89 million light-years distant from Earth. Although this may seem far away, it exceeds the previous record of 470 million light-years by quite a margin, making this new pair of supermassive black holes the closest to us yet.

Supermassive black holes lurk in the center of massive galaxies, and when two of those galaxies merge, the black holes end up on a collision course. The NGC 7727 pair broke the record for the smallest separation between two supermassive black holes, as they are observed to be only 1600 light-years apart in the sky. “This is the first time that we have found two supermassive black holes that are so close to each other, less than half the separation of the previous record holder,”

“The small separation and speed of the two black holes indicate that they will merge into a monstrous black hole, probably within the next 250 million years,” adds co-author Holger Baumgardt, a professor at the University of Queensland, Australia. The merger of black holes like these could explain the process of formation of the most massive black holes in the Universe.

Voggel and his team were able to determine the masses of the two objects by observing how the gravitational pull of black holes influences the motion of the stars around them. The largest black hole, located right in the core of NGC 7727, was found to have a mass almost 154 million times that of the Sun, while its companion has 6.3 million solar masses.

It is the first time that masses have been measured in this way in the case of a pair of supermassive black holes. This feat was possible thanks to the proximity of the system to the Earth and the detailed observations that the team obtained at the Paranal Observatory, in Chile, using the MUSE (Multi-Unit Spectroscopic Explorer) instrument, installed in the VLT of ESO, an instrument that Voggel learned to work with during his time as a student at ESO. Measuring the masses with MUSE, and using additional data from the NASA / ESA Hubble Space Telescope, allowed the team to confirm that the objects at NGC 7727 were, in fact, supermassive black holes.

The astronomical community suspected that the galaxy was home to two black holes, but they had not been able to confirm their presence until now, as we do not see large amounts of high-energy radiation coming from their immediate surroundings, which would otherwise give them away. “Our finding implies that there could be many more of these relics from galaxy mergers out there and they may contain many hidden massive black holes that are still waiting to be found,” says Voggel. “The total number of known supermassive black holes in the local universe could increase by 30 percent.”