X's may mark spots where black holes merge. (Cosmic Twist).

In deep space, black holes may sometimes swirl together and then collide, sending out staggering bursts of gravitational waves. Scientists are developing sophisticated new instruments to measure such never-before-detected waves--which would be ripples in space-time itself (SN: 1/8/00, p. 26). In the meantime, they're wondering whether some evidence for these magnificent events has been under their noses for years.

As it turns out, such titanic encounters sometimes leave behind a kind of celestial graffiti, a new study suggests. Ever since 1978, when Pon D. Ekers of the Australia Telescope National Facility in Epping and his colleagues first found a vast X-shaped source of radio waves centered on galaxy NGC 326, a growing roster of such crisscross formations has puzzled observers. In the past 2 decades, radioastronomers have identified about a dozen such sources.

Now, Ekers and David Merritt of Rutgers University in New Brunswick, N.J., propose that a supermassive black hole, like a bizarre spray-paint can with nozzles on opposite sides, spends roughly a billion years spewing jets of matter and radiation along one line in space. Those ejections form plumes of searing plasma hundreds of thousands of light-years long.

Eventually, Ekers and Merritt suspect, a collision between a massive jet-shooting black hole and a second, smaller black hole jars the bigger black hole's twin jets out of line. In the millions of years that follow, the reoriented jets make new plumes that form the second stroke of an X.

It would take a mighty wallop to tip the axis of a massive black hole. Indeed, Merritt notes, collisions between supermassive black holes are expected to be "the most energetic events in the universe."

Today, most astronomers think that the core of every galaxy harbors a supermassive black hole weighing a million to a billion times the mass of our sun (SN: 2/23/02, p. 122). Of these black holes, about 1 percent produces jets. Because galaxies are known to collide with each other, scientists suspect the galaxies' black holes also crash together sometimes.

Lacking observational evidence of such mergers, scientists have been trying to simulate them on computers. In such studies by Merritt and others, however, pairs of faux black holes only reach within about a light-year of each other before their approach stalls. Those results have made the idea of colliding black holes seem less realistic.

In an upcoming issue of Science, Merritt and Ekers outline their new interpretation of X-type radio sources. If it is correct, Merritt claims, these sources would amount to "smoking-gun evidence that black holes do actually merge in the universe."

Donald C. Backer, a radioastronomer at the University of California, Berkeley, says that Merritt and Ekers have offered "solid" arguments for their interpretation and "have brought together important theoretical and observational perspectives."

"It is an important finding if it is confirmed," adds Qingjuan Yu of Princeton University. She also notes that scientists continue to come up with other explanations for the X shapes.