Closest pair of supermassive black holes headed for monster merger
Astronomers have discovered the closest pair of supermassive black holes yet known – and that record has two meanings. Not only are they the closest pair to Earth, but they’re the closest to each other as well, barreling towards each other for an eventual monster merger.
The two black holes are located in a galaxy called NGC 7727, about 89 million light-years from Earth. That might sound like a long way, but in cosmic terms it’s a mere stone’s throw, and far closer than the previous record holders at 470 million light-years. Of course, the closest single supermassive black hole is Sagittarius A*, which lurks at the center of the Milky Way just 26,000 light-years away.
But perhaps even more intriguing is the proximity between the pair. These two supermassive black holes are just 1,600 light-years apart, less than half the distance between the previous record holders. And they’re getting closer.
“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,” says Holger Baumgardt, co-author of the study.
The larger of the two is located right at the center of the galaxy, boasting a mass of almost 154 million times that of the Sun. The second is a comparatively petite 6.3 million solar masses, and the astronomers believe that it was originally the core of another galaxy that merged with NGC 7727 about a billion years ago.
Such a collision had long been suspected. NGC 7727 is a very irregular galaxy, with amorphous spiral arms and strange streams of stars in its outer regions. The two bright objects near the center were thought to be supermassive black holes, but it hadn’t been confirmed.
So for the new study, the researchers set out to measure the mass of the objects. They used the Multi-Unit Spectroscopic Explorer (MUSE), an instrument on the Very Large Telescope (VLT) in Chile, to study the gravitational influence the objects exerted on the stars around them. This revealed that the objects had masses of many millions of Suns, leaving supermassive black holes as the only candidates.
“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,” says Karina Voggel, lead author of the study. “It could increase the total number of supermassive black holes known in the local Universe by 30 percent.”
The research was published in the journal Astronomy & Astrophysics. The team describes the work in the video below.
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