Supermassive Black Holes Spotted in Photo of Andromeda Are About to Collide in Megamerger

Two supermassive black holes recently spotted in a picture of the Andromeda galaxy are the closest orbiting pair ever discovered—and could be on course for a catastrophic cosmic collision in as little as 350 years.

Andromeda is our closest neighboring galaxy, sitting around 2.5 million light-years away: fairly close, in astronomical terms. It is expected it will collide with our own home galaxy, the Milky Way, about 4 billion years from now.

Scientists from the University of Washington were studying an unusual object within Andromeda dubbed J0045+41, which they thought to be a special type of star. Another team of scientists had previously identified it as a pair of stars orbiting each other once every 80 days, but the UW team thought something more was going on.

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The Andromeda galaxy. The blue region is known as J0045 41. NASA/CXC/University of Washington/ESA

"We were looking for a special type of star in M31 and thought we had found one," said Trevor Dorn-Wallenstein, lead author on the study published in the Astrophysical Journal. "We were surprised and excited to find something far stranger!"

To observe it, they used NASA's Chandra X-ray Observatory and two ground-based telescopes in Hawaii and California. Their observations showed the object could not be a star in the Andromeda galaxy; the intensity of the X-rays meant it must either be a binary system containing a neutron star or black hole, or that it was a type of distant system containing at least one supermassive black hole.

Further analysis of the object showed there were in fact two supermassive black holes present, and that they were orbiting one another at an incredibly close distance. Researchers estimated that the pair were separated by a distance of one hundredth of a light-year—just a few hundred times the distance between Earth and the sun—and that they orbited each other at periods varying between 80 and 320 days.

The two black holes are currently drawn toward each other and will at some point collide to become one. "We're unable to pinpoint exactly how much mass each of these black holes contains," co-author John Ruan said in a statement. "Depending on that, we think this pair will collide and merge into one black hole in as little as 350 years or as much as 360,000 years."

"J0045+41 is an exciting and unique object," the team wrote, adding that follow-up observations were required to confirm they were indeed looking at a supermassive black-hole binary system. A closer look would also help them confirm and narrow down the orbital periods estimated.