Astronomers have spotted two supermassive black holes in the crowded center of a distant galaxy. And it's only a matter of time, they say, before the pair collide.

Nov. 20, 2002: For the first time, scientists have found proof of two supermassive black holes together in the same galaxy. These black holes are orbiting each other and will merge several hundred million years from now. The event will unleash intense radiation and gravitational waves ... and leave behind an even larger black hole than before.

NASA's Chandra X-ray Observatory spotted the two black holes in the galaxy NGC 6240. The observatory was able to "see" them because the black holes are surrounded by hot swirling vortices of matter called accretion disks. Such disks are strong sources of x-rays.

Above: An artist's concept of two black holes (surrounded by glowing hot accretion disks) about to merge.

"The breakthrough came with Chandra's ability to clearly distinguish the two nuclei and to measure the details of their X-radiation," said Guenther Hasinger of the Max Planck Institute for Extraterrestrial Physics in Germany. He's a coauthor of an upcoming paper in the Astrophysical Journal Letters describing the research.

At a distance of about 400 million light-years, NGC 6240 is a prime example of a starburst galaxy--that is, 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. Optical, radio, and infrared observations had previously revealed two bright nuclei, but their nature was a mystery.

X-rays, however, can penetrate the veil of gas and dust. "With Chandra, we hoped to determine which one, if either, of the nuclei was an active supermassive black hole," said Stefanie Komossa, also of the Max Planck Institute, lead author of the paper on NGC 6240. "Much to our surprise, we found that both were active black holes!"

Left: A composite optical (Hubble) and x-ray (Chandra) photo of the inner regions of galaxy NGC 6240. X-rays from the two supermassive black holes appear blue in this false-color image.

"The detection of a binary black hole supports the idea that black holes can grow to enormous masses in the centers of galaxies by merging with other black holes," said Komossa. "This is important for understanding how galaxies form and evolve," she said.

Over the course of the next few hundred million years, the two black holes in NGC 6240, which are about 3000 light-years apart, will drift toward one another and merge to form an even larger supermassive black hole. Toward the end of this process an enormous burst of gravitational waves will be produced.

These gravitational waves will spread through the universe and produce ripples in the fabric of space, which would appear as minute changes in the distance between any two points. NASA's planned space-based detector, LISA (Laser Interferometer Space Antenna), will search for gravitational waves from massive black-hole mergers. These events are estimated to occur several times each year in the observable universe.

"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," said Hasinger.