14 DAY TRIAL // Black holes grow by swallowing various things, but sometimes they even swallow each other and merge into a giant black hole. Astronomers cannot directly see a black hole because nothing comes from a black hole. However, astronomers can see things falling inside the black hole, and, more importantly, they can see X-rays ejected from the gas that's falling to the black hole and that heats up in the process. As the black hole moves through the gas cloud, and matter falls toward the black hole, a pair of jets of X-rays is ejected backward.
"The jets are similar to the contrails produced by planes as they fly through the air on Earth," said Craig Sarazin of the University of Virginia. "From the contrails, we can determine where the planes have been, and in which direction they are going."
Astronomers have observed several pairs of black holes, and now it is certain that one of these pairs, located at the center of a galaxy cluster called Abell 400, is bound for a collision. "The question was: Is this pair of super-massive 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." They now know this because "the jets are bent together and intertwined, which indicates that the pair of super-massive black holes are bound and moving together."
The X-ray jets have been observed with NASA's Chandra X-ray Observatory.
However, the actual collision will not happen earlier than several million years from now. According to the general theory of relativity, this event will produce a burst of gravitational waves.
Picture: Composite X-ray/radio image of the galaxy cluster Abell 400; the two bright spots in the image are the super-massive black holes, while the trails are the jets of superheated gas emanating from the vicinity of the black holes. Credit: NASA / NRAO