Like pirates poring over a yellowed map, astrophysicists sometimes find that X marks the spot of hidden treasure. To David Merritt of Rutgers University in New Jersey, X-shaped formations around some galaxies indicate the likely site of the collision of two supermassive black holes—a crash that would reverberate across the cosmos.
Merritt, along with radio astronomer Ron Ekers of the Australia Telescope National Facility in Sydney, looked at high-resolution radio images of 11 galaxies having unusual lobes of radio-emitting gas that point in skewed directions. These lobes probably formed when matter circling a black hole squirted out as jets along the hole's axis of rotation. But the X-shaped lobes could form only if the jets shot out first along one axis, then along another. In other words, something must have caused the black hole to flip over. There is just one thing powerful enough to do that: another supermassive black hole.
Conditions are ripe for disaster when two galaxies plow into each other. Stars would survive, merely settling into new orbits through the combined galaxy. For black holes, however, gravitational interactions could make a collision inevitable. "These mergers, which happen about once a year in the universe, are the most energetic events you can imagine," Merritt says. In just one minute, according to computer simulations, two regions of warped space meet and absorb each other, creating a larger black hole with a tilted spin axis. Torrents of energy spew out, presumably in the form of gravity waves. So far, we have only indirect evidence of such mergers, but the LIGO detectors in Louisiana and Washington State may be able to pick up their gravitational screams.
Galaxy NGC 326, seen in radio waves, has two sets of jets. The central source (inset) is probably powered by a black hole. Photograph courtesy of Matteo Murgia.