Illustration by Christoph Niemann

The Paper Tracing Black Hole Mergers Through Radio Lobe Morphology

The Journal Science, August 23, 2002, Vol. 297

The Author David Merritt and R.D. Ekers

The Gist What do you get when black holes meet? A really big black hole.

Before Translation If the coalescence rate of binary SBHs is comparable to the galaxy merger rate, then the binary separation must be able to drop from 1 to 0.01 pc in a time shorter than 1 Gy. The predicted event rate for gravitational wave interferometers should then be about equal to the integrated galaxy merger rate out to a redshift z ≈ 5, implying a time between detections of 1 year.

Ravenous and unbelievably dense, a supermassive black hole is one of the most fearsome creations in the universe. With the mass of 100 million suns, these black holes swallow all the galactic material in their neighborhoods. But what would happen if two of these cosmic monstrosities actually bumped into one another? Astronomers David Merritt of Rutgers University and R.D. Ekers of the University of California, Berkeley, think they know.

Black holes meet when gravity pulls two galaxies together over billions of years, forming a gargantuan new system. Each black hole starts in the center of its host galaxy and drifts toward the middle of the new one. As they draw closer together, the black holes begin to orbit one another. In most computer simulations, this dance lasts indefinitely, with each black hole keeping its distance. But according to Merritt, in some cases the black holes can lose energy, gradually causing their orbits to shrink. Eventually, after hundreds of millions of years, the smaller of the two plunges inward, coalescing with its larger partner. Merritt compares it to the merger of two raindrops: Each black hole loses its original identity, forming a new, more massive whole.

Merritt and Ekers think they’ve found evidence of such mergers in a group of unusual X-shaped radio signals coming from distant galaxies. These X formations, they say, are proof of rapidly shifting plasma jets-streams of superheated gas shooting out from a black hole’s center like sprouting palm trees.

Although a supermassive black hole is itself invisible, a plasma jet is one indication of its presence. Another sign is an accretion disc, a collection of swirling gas and dust sometimes likened to tree-swinging monkeys. The plasma palm trees extend out perpendicular to this disc, in line with the black hole’s spin axis. Several light-years from the center, the superheated gas in the jet starts to cool and expand. As it does so, a radio beam shoots out from the jet, like a giant flashlight shining through space.

Astronomers were recently able to pick up some of these radio beams but were puzzled by their criss-cross formations. A movement of the jet seemed to be an obvious explanation. The problem was, a plasma jet-and its radio beams-should be extremely tough to move. A force acting gradually on the black hole, like a long-term gravitational pull, might produce an S-curved jet, but the X shape suggested rapid change. To produce that, you would have to knock around the black hole itself-and spinning objects that size are nothing if not stable.

Merritt and Ekers argue that the only plausible explanation for this intense result is the meeting of two supermassive black holes. And these exciting events can happen quickly. According to Merritt, once the smaller black hole begins to dive into its partner, it takes only about a year for the jet to shift. Even more spectacular, when the two black holes finally coalesce, there is a huge burst of gravitational radiation that could last less than 2 minutes. Merritt hopes that NASA’s proposed Laser Interferometer Space Antenna (LISA), a space-based gravity wave detector not yet slated for construction, could pick up one of these merger events every year. Aside from proving Merritt and Eker’s theory, such evidence would constitute a solid proof of Albert Einstein’s general theory of relativity.