Hubble Space Telescope showing a supermassive black hole
NASA, ESA, M. Chiaberge (STScI/ESA)

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Are gravitational waves kicking this black hole out of its galaxy?

Astronomers have just spied a black hole with a mass 1 billion times the sun’s hurtling toward our galaxy. But scientists aren’t worried about it making contact: It’s some 8 billion light-years away from Earth and traveling at less than 1% the speed of light. Instead, they’re wondering how it got the boot from its parent galaxy, 3C186 (fuzzy mass in the Hubble telescope image, above). Most black holes lie quietly—if voraciously—at the center of their galaxies, slurping up the occasional passing star. But every once in a while, two galaxies merge, and the black holes in their centers begin to swirl around each other in a pas des deux that eventually leads to a devastating merger. The wandering black hole (bright spot above), may be the result of one such merger. Based on the wavelengths of spectral lines emitted by the luminous gas surrounding the black hole, the object is traveling at a speed of about 7.5 million kilometers per hour—a rate that would carry it from Earth to the moon in about 3 minutes. If the most likely scenario is true, then a massive kick from the merger of two black holes some 1.2 billion years ago would have created a ripple of gravitational waves, the researchers suggest in a forthcoming issue of Astronomy & Astrophysics. And if the precollision black holes didn’t have the same mass and rotation rate as each other, the waves would have been stronger in some directions than others, giving the resulting object a jolt equivalent to the energy of 100 million supernovae exploding simultaneously, the researchers estimate. Other runaway black holes have been proposed, but none of them has yet been confirmed.