Giant black holes to collide, astronomers say
and World Science staff
Two distant “supermassive” black holes are spiraling into each other, astronomers say—and to a violent merger that will spawn an even bigger “super-supermassive black hole,” capable of swallowing billions of stars.
Black holes are objects so dense and massive that nothing can escape their gravity’s pull, not even light. “Supermassive black holes” are the super-sized versions of these objects. Containing the weight of billions of stars crammed into a small space, they continually suck in more stars, further building their mass and gravitational pull.
“Black holes are the ultimate garbage disposals,” said Craig Sarazin of the University of Virginia in Charlottesville, Va., one of the researchers. “The material they swallow disappears without any trace, except for the [increased] gravity of the black hole.”
Black holes are also among the oldest objects in the cosmos, and are thought to hold clues to understanding the formation and development of the universe.
Astronomers have theorized the existence of “coupled” black holes that orbit each other, drawn together by each other’s gravity. They would eventually collide and unite. But evidence for such events is limited. The study provides such corroboration, according to the astronomers, who published their findings in the April 6 issue of the research journal Astronomy & Astrophysics.
“The two key questions about supermassive black holes are: Where do they come from and how do they grow over time?” said Sarazin. “The birth, care and nurturing of supermassive black holes is a very active area of study.” The researchers used NASA’s orbiting Chandra X-ray Observatory to glean their results.
Black holes are observable only because as things fall into them, they heat up and spew X-rays, which are invisible to our eyes but detectable with X-ray telescopes. “There is no way to determine how a black hole was created or what kinds of things it has swallowed just by looking at the resulting black hole,” Sarazin said. “You have to catch [it] when it is sitting down to dinner or still eating.”
His group watched the center of a cluster of galaxies named Abell 400 where astronomers had suggested two supermassive holes might be colliding. They seemed to be close together, but it was unclear that they were gravitationally bound or merging.
“The question was: Is this pair of supermassive black holes an old married couple, or just strangers passing in the night?” Sarazin said. “We now know that they are coupled, but more like the mating of black widow spiders. One of the black holes invariably will eat the other.”
NASA plans to build three space satellites collectively called the Laser Interferometry Space Antenna (LISA) to detect gravity waves from merging black holes, an exotic type of radiation predicted be Einstein. Astronomers want to be sure the supermassive mergers really do happen “so that LISA will have something to detect,” Sarazin said.
In recent years, astronomers have found that every large galaxy likely has a supermassive black hole at its core. The Milky Way’s own has swallowed as much material as four million suns. The biggest galaxies contain black holes that have swallowed many billions of stars’ worth of matter.
Two galaxies containing supermassive black holes can collide and merge. Eventually their two supermassive black holes would fall to the center of the merged galaxy and spiral together, eventually uniting.
Sarazin’s team reported that the two merging holes seem to be swallowing gas from their host galaxy. Each is ejecting a pair of oppositely-directed jets of plasma, an electrified fluid of subatomic particles. As the holes fall through the surrounding gas, these jets, emitting radio waves, are swept back behind them.
“The jets are similar to the contrails produced by planes as they fly through the air on Earth,” Sarazin said. “From the contrails, we can determine where the planes have been, and in which direction they are going. What we see is that the jets are bent together and intertwined, which indicates that the pair of supermassive black holes are bound and moving together.”
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