An Astronomical First: Binary Black Holes Devouring a Star in an Inactive Galaxy

The celestial oddity was found because astronomers happened to catch the black holes ripping a star apart.

Amy Shira Teitel

An artist's render of a pair of black holes, with the one on top accreting material from a dying star, while the one on bottom breaks the stream of debris. Image: ESA/C. Carreau

There’s a fair bit of luck involved in astronomy—not in the science, but in the spotting. Sometimes you simply have to be looking in the right place at the right time to see something amazing that changes the way we understand the universe. Case in point: astronomers recently found a pair of black holes orbiting one another at the centre of an inactive galaxy. The celestial oddity was found because astronomers happened to catch the black holes ripping a star apart.

The stellar death and black hole pair was spotted by the European Space Agency’s XMM-Newton space-based observatory, and the results are soon to be published in the Astrophysical Journal. (They're also currently on arXiv.) Just finding the black holes is quite an accomplishment for the joint Chinese-German research team: Their galaxy is anonymously-named SDSS J120136, which sits about 2 billion light years away. But beyond just finding the dueling black holes, the real feat is in discovering the implications of their mere existence.

Astronomers usually expect to find a supermassive black hole at the center of galaxies. But finding a galaxy with two black holes is unique, and is evidence that two galaxies actually collided and combined into one. Knowing that can give astronauts a way of reverse engineering the new galaxy’s story; in essence, binary supermassive black holes could shed light on how galaxies evolve to their current day shapes and sizes.

Binary black holes aren’t unheard of, though they are rare. Only a few candidate binary supermassive black hole pairs have been found, but they've only been identified in active galaxies. Binary black holes create an environment where dust clouds are constantly being torn apart and crushed out of existence. A byproduct of this violent environment is light; the gas clouds heat as they’re ripped apart, and that heat is broadcast as light across many wavelengths including X-rays. 

This means the galaxies have very brightly shining cores, which in turn leads astronomers to believe they’re active. But the latest pair was found in an inactive galaxy, so it’s changing what we know about binary black holes.

“There might be a whole population of quiescent galaxies that host binary black holes in their centres,” said co-author Stefanie Komossa, a researcher at the Max Planck Institute for Radio Astronomy in Bonn, Germany, in a release.

Finding these quiet galaxies with binary black holes is quite difficult since there’s no gas to glow as it is ripped apart. Instead, astronomers can hope to catch what’s called a ‘tidal disruption event’—a star being ripped apart—in one of these quiet galaxies. A star emits X-rays much as a cloud of gas as it dies at the hands of a black hole.

ESA’s XMM-Newton looks at X-rays in distant galaxies, and it’s a clever move that let it find the binary black holes in a quiet galaxy. XMM-Newton first caught the tidal disruption event on June 10, 2010. Days later, the galaxy was still shooting out X-rays for telescopes to measure. The X-rays became undetectable 27 days after the initial find, reappeared 21 days later, then faded again.

“This is exactly what you would expect from a pair of supermassive black holes orbiting one another,” said lead author Fukun Liu, a researcher from Peking University in China, in a release.

Working with models of binary black holes, Liu has predicted exactly what he saw with galaxy J120136. His models suggest that the two black holes lie within about 1/2000th of a lightyear apart, which is roughly the width of our solar system. And being this close, they likely won’t be individual black holes for long. They will spiral together, radiating energy, until they merge into a single black hole about two million years from now.

After this chance discovery, astronomers will look a little more closely at quiet galaxies to see if maybe there isn’t a binary black hole pair at its core. These are the kinds of finds that really change the way we think about the universe, and open up whole new areas for astronomers to study.