The closest pair of supermassive black holes to Earth ever observed so far has been identified: it is located in the galaxy NGC 7727 in the constellation Aquarius, about 89 million light years from Earth, much less than the previous record of 470 million light years.
The discovery, made thanks to the Vlt (Very Large Telescope) of the European Southern Observatory (Eso) in Chile, is published in Astronomy & Astrophysics by an international team led by Karina Voggel, astronomer of the Strasbourg Observatory in France.
Supermassive black holes lurk in the center of massive galaxies, and when two of them merge, the black holes end up on a collision course. The pair in NGC 7727 broke the record for the smallest separation between two supermassive black holes, only 1,600 light-years apart, "less than half the previous record," Voggel points out. "The small separation and speed of the two black holes indicate that they will merge into a gigantic black hole, probably within the next 250 million years," adds Holger Baumgardt of the University of Queensland, Australia. The merger of black holes like these could explain how the most massive black holes in the universe came into being.
The researchers determined the mass of the two objects by observing how the gravitational pull of black holes affects the motion of the stars around them. The largest black hole, at the center of NGC 7727, was found to have a mass equal to almost 154 million times that of the Sun, while the mass of the companion is 6.3 million solar masses. It is the first time that mass has been measured in this way in a pair of supermassive black holes: a feat made possible by the system's proximity to Earth and by the observations obtained with the Muse (Multi-Unit Spectroscopic Explorer) instrument installed on Vlt. The discovery "implies that there may be many more remnants of galaxy mergers out there and may contain many hidden massive black holes still waiting to be found," says Voggel. "It could increase the total number of known supermassive black holes in the local universe by 30%. "