Supermassive black hole pair on path to collision

Two supermassive black holes  have been spotted on their way to a cataclysmic collision. The newly discovered pair are the closest to colliding of any SMBH pair ever observed.

Findings in the Astrophysical Journal Letters say the pair is around 750 light years apart. This means they won’t actually crash into each other for another few hundred million years.

For comparison, our nearest star, Proxima Centauri, is only four light years from Earth (nearly 200 times shorter in distance), and famous star Betelgeuse is a little over 700 light years away from us.

Like the SMBH in the centre of our Milky Way galaxy, the SMBH pair are each the central black holes of two galaxies which are in the midst of a galactic merger.

Between them, the pair of gargantuan black holes have a combined mass of around 325 million times that of our Sun. As their host galaxies continue into the final stages of their merger, the two black holes will eventually begin circling each other, spiralling closer together until they eventually form one truly monstrous black hole.

This artist’s conception shows a late-stage galaxy merger and its two newly-discovered central black holes. Credit: ALMA (ESO/NAOJ/NRAO), M. Koss et al (Eureka Scientific), S. Dagnello (NRAO/AUI/NSF).

The astronomers, announcing their discovery at an American Astronomical Society meeting in Seattle on January 9, believe the finding will help estimate how many other SMBHs elsewhere in the universe are coming close to a collision.

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Such improved surveys of SMBH collisions will aid in efforts to detect gravitational waves – ripples in the very fabric of spacetime caused by intense gravitational events.

Study co-author Chiara Mingarelli, a research scientist at the Flatiron Institute’s Center for Computational Astrophysics in New York City, says the short distance between the SMBHs “is fairly close to the limit of what we can detect, which is why this is so exciting.”

Astronomers were forced to use seven different telescopes to differentiate between the two goliaths, including NASA’s Hubble Space Telescope. The bright stars, and luminous gas and dust surrounding the black holes makes them undetectable directly using optical telescopes.

But the pair was found soon after the search commenced, suggesting close-together SMBHs “are probably more common than we think, given that we found these two and we didn’t have to look very far to find them,” Mingarelli adds.

This suggests to Mingarelli and her team that the first ever detection of gravitational wave “background babble” from all the SMBH collisions going on in the universe may come “very soon.”

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Previously, observations of merging galaxies showed only one SMBH because the central black holes are too close together to tell them apart using one telescope.

Telescope observations of two newly discovered supermassive black holes on a collision course. Their host galaxy, left, is a mash-up of two galaxies that have collided. The pink box shows the location of the supermassive black holes. Close observation of the pair, right, reveals two distinct black holes (white spots) only 750 light-years apart. Credit: M.J. Koss et al.

The new survey of galactic mergers combined 12 observations made using seven different telescopes on Earth and in orbit.

“It’s important that with all these different images, you get the same story – that there are two black holes,” says Mingarelli. “This is where other studies [of close-proximity supermassive black holes] have fallen down in the past. When people followed them up, it turned out that there was just one black hole. [This time we] have many observations, all in agreement.”

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