April 17, 2006— Astronomers have spotted the gargantuan spiraling contrails of two super-massive black holes that are on a mega gigantic collision course.

The discovery is important because it provides evidence of one of the ways that the most gigantic black holes are thought to form: by merging. A report on the finding is published in the current issue of Astronomy & Astrophysics.

The billion-star devouring black holes are the first to be found on such a cosmically cataclysmic death spiral and may help explain how the most monstrous black holes in the universe are created: by cannibalism.

"This was an accidental discovery," said astronomer Craig Sarazin of the University of Virginia.

Sarazin and his colleagues had been studying a dense super cluster of galaxies called Abell 400 with the orbiting Chandra X-Ray Observatory and the Very Large Array radio telescope when the strange dual black hole popped out at them. "What we detected were the X-rays from two super-massive black holes in the process of merging," Sarazin said.

The X-rays are caused by jets of superheated material streaming from the two giant black holes, said Sarazin. The intertwined contrails of the jets, made by buffeting the interstellar gases there, were detected in radio waves.

Both black holes are so large they could only have been created at the centers of two galaxies, just like the supermassive black hole at the center of the Milky Way.

There are two types of black holes, explained Sarazin. One type is made by the collapse of a star at least 50 times the mass of our sun (a.k.a. 50 solar masses). The other type is the far more massive giants at the centers of galaxies, which range from a million to ten billion solar masses.

Of the giant sort, it's thought that they are made by either growing slowly, via constantly gobbling up other stars and galactic material, or they merge with other giants when galaxies collide.

The problem is, galactic collisions are relatively short events, so it's hard to catch it happening, said black hole researcher Milos Milosavljevic of the California Institute of Technology.

"These things are very hard to see," said Milosavljevic.

In the case of Abell 400, the two black holes are still far enough apart to observe, far enough to fit the entire Milky Way between them.

The two black holes would stay apart and theoretically orbit each other forever if it were not for the many stars in their midst, he said. Each time the gravity of a black hole slingshots a star out of the galaxy instead of eating it — a very common event — the black hole gives up some of its velocity and edges nearer the other black hole, explained Milosavljevic.

When the two come to within about a single light year of each other, depending on their masses, they begin losing energy by sending out gravity waves, he said.

That's good news to researchers hoping to detect gravity waves. The discovery of Abell 400 black holes means there's a good chance other such collisions are out there and much further along, he said.

This sort of gravity wave requires a very large gravity telescope composed of three small spacecraft connected by laser beams, said Milosavljevic.

Just such a gravity observatory has already been designed by NASA, the Laser Interferometry Space Antenna, but is awaiting funding.