ESO telescopes reveal closest pair of supermassive black holes yet seen

Close-up view of the two bright galactic nuclei, each housing a supermassive black hole, in NGC 7727, a galaxy located 89 million light-years away from Earth in the constellation Aquarius. Each nucleus consists of a dense group of stars with a supermassive black hole at its center. The image was taken with the MUSE instrument on ESO’s Very Large Telescope (VLT) at the Paranal Observatory in Chile. Credit: ESO/Voggel et al.

Located in the galaxy NGC 7727 in the constellation Aquarius, the supermassive black hole pair is about 89 million light-years away from Earth. Although this may seem distant, it beats the previous record of 470 million light-years by quite some margin, making the newfound supermassive black hole pair the nearest us yet.  

Supermassive black holes lurk at the center of massive galaxies and when two such galaxies merge, the black holes end up on a collision course. The pair in NGC 7727 beat the record for the smallest separation between two supermassive black holes, as they are observed to be just 1,600 light-years apart. “It is the first time we find two supermassive black holes that are this close to each other, less than half the separation of the previous record holder,” said Karina Voggel, an astronomer at the Strasbourg Observatory in France and lead author of the study published online in Astronomy & Astrophysics.

“The small separation and velocity of the two black holes indicate that they will merge into one monster black hole, probably within the next 250 million years,” added co-author Holger Baumgardt, a professor at the University of Queensland, Australia. The merging of black holes like these could explain how the most massive black holes in the Universe come to be.

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

It is the first time the masses have been measured in this way for a supermassive black hole pair. This feat was made possible thanks to the close proximity of the system to Earth and the detailed observations the team obtained at the Paranal Observatory in Chile using the Multi-Unit Spectroscopic Explorer (MUSE) on ESO’s Very Large Telescope, an instrument Voggel learnt to work with during her 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 in NGC 7727 were indeed supermassive black holes.

Astronomers suspected that the galaxy hosted the two black holes, but they had not been able to confirm their presence until now since 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 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 Voggel. “It could increase the total number of supermassive black holes known in the local Universe by 30 percent.”

The NGC 7727 galaxy, shown in this image from ESO’s VLT Survey Telescope (VST) in Chile. Located 89 million light-years away from Earth in the constellation Aquarius, NGC 7727 is believed to be the result of a clash between two galaxies that occurred about one billion years ago. The consequences of this tremendous cosmic bump are still evident in the peculiar, irregular shape of NGC 7727 and the streams of stars in its outer regions.  Credit: ESO/VST ATLAS team. Acknowledgement: Durham University/CASU/WFAU

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