Inside most galaxies a supermassive black hole lurks. But one galaxy about 4 billion light-years from us was recently discovered to have not one, not even two, but three gigantic black holes at its center.
Such triple systems appear to be extremely rare—only four are known. The newfound system includes two black holes orbiting each other very closely, about 450 light-years apart, with a third black hole a bit farther out. The central pair zoom around each other at a fast clip, about 300 times the speed of sound on Earth. The hole trinity also represents the tightest trio of black holes known to date. It’s described in the journal Nature. [R.P. Deane et al, A close-pair binary in a distant triple supermassive black hole system]
As these objects continue to orbit at the center of their galaxy, gravity will eventually pull them closer and closer together. Ultimately, they may even merge. Researchers hope this triple-black-hole system may be a good place to look for space-time ripples called gravitational waves. As their orbits shrink, the black holes should radiate away some of their orbital energy as the sought-after gravitational waves, predicted by Einstein a century ago.
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[Scientific American is part of Nature Publishing Group.]