Black holes on collision course

For the first time scientists have seen two supermassive black holes existing together at the core of the same galaxy.

The black holes are orbiting each other and will collide and merge to create an even larger black hole - resulting in a catastrophic event that will unleash intense radiation and gravitational waves.

However, this will not happen for several hundred million years.

The observations were made by the Chandra X-ray Observatory. They show the nucleus of an extraordinarily bright galaxy, known as NGC 6240, contains two giant black holes, both taking material from their surroundings.

This discovery shows that massive black holes can grow through mergers in the centres of galaxies.

Cosmic fingerprints

"The breakthrough came with Chandra's ability to clearly distinguish the two nuclei, and measure the details of the X-radiation from each nucleus," says Guenther Hasinger, of the Max Planck Institute for Extraterrestrial Physics in Germany.

"These cosmic fingerprints revealed features characteristic of supermassive black holes - an excess of high-energy radiation from gas swirling around a black hole," he says.

Previous X-ray observatories had shown that the central region of NGC 6240 produces X-rays, while radio, infrared and optical observations had detected two bright nuclei at the centre of the galaxy, but the nature of this region was a mystery.

Astronomers did not know the location of the X-ray source, or the nature of the two bright nuclei.

"With Chandra, we hoped to determine which one, if either, of the nuclei was an active supermassive black hole," says Stefanie Komossa, also of the Max Planck Institute.

"Much to our surprise, we found that both were active black holes."

Cosmic collision

At a distance of about 400 million light-years, NGC 6240 is a prime example of a massive galaxy in which stars are forming at an exceptionally rapid rate due to a recent collision and subsequent merger of two smaller galaxies.

Because of the large amount of dust and gas in such galaxies, it is difficult to peer deep into their central regions with optical telescopes. However, X-rays emanating from the galactic core can penetrate the veil of gas and dust.

"The detection of a binary black hole supports the idea that black holes can grow to enormous masses in the centres of galaxies by merging with other black holes," said Komossa.

"This is important for understanding how galaxies form and evolve," she says.

Over the course of the next few hundred million years, the two black holes in NGC 6240, which are about 3,000 light-years apart, will drift towards one another and merge to form an even larger supermassive black hole.

Towards the end of this process, an enormous burst of gravitational waves.

"This is the first time we see a binary black hole in action, the smoking gun for something that will become a major gravitational wave burst in the future," says Hasinger.