Millions of years ago, B3 1715+425 was just an ordinary supermassive black hole. It had a comfortable life, of devouring stars and belching deadly X-rays, at the centre of its distant galaxy. Now, starless and alone, it's screaming through space at 3219km per second — and it may never stop.
Artist's conception of how the "nearly naked" supermassive black hole originated. Image: Bill Saxton, NRAO/AUI/NSF
BC 1715+425's troubles began when its galaxy bumped up against another. This isn't all that unusual: In fact, astronomers believe that the largest galaxies in our universe formed during ancient mergers. Normally, when two galaxies collide, the supermassive black holes at their centres start to orbit one another, moving closer and closer together in an inescapable gravitational attraction.
Eventually, those black holes can fuse, releasing a burst of energy as gravitational waves and completing the cosmic joining.
Most of the time, this process seems to work out for all parties involved, judging from the fact that nearly all supermassive black holes reside at the centre of galaxies, and nearly all galactic centres contain a supermassive black hole. But every now and then, something goes wrong and cosmic wreckage ensues. B3 1715+425, speeding away from the core of a bloated galactic merger two billion light years from Earth, is living proof of this.
The strange object was discovered by astronomers using the Very Long Baseline Array (VLBA), a network of 10 25m dish antennas located around the world and operated out of New Mexico.
"We were looking for orbiting pairs of supermassive black holes, with one offset from the center of a galaxy, as telltale evidence of a previous galaxy merger," said James Condon, the astronomer at the National Radio Astronomy Observatory who led the study. "Instead, we found this black hole fleeing from the larger galaxy and leaving a trail of debris behind it."
The working theory is that millions of years ago, B3 1715+425's galaxy passed through a much larger galaxy (one that had formed during many previous mergers) and got shredded to bits, a bit like a paper aeroplane flying into a hurricane. The leftovers include a faint galactic remnant, just 3000 light years across, and the supermassive black hole itself, nearly naked and haemorrhaging ionised gas as it tears through the void.
Astronomers believe the cosmic carnage will become invisible in about a billion years, as the galactic remnant ceases to form new stars.
"We've not seen anything like this before," Condon said.
Have other galaxies and supermassive black holes met the same unfortunate fate? It seems likely. Condon and his team will continue to investigate the matter using the VLBA, and with high-resolution optical telescopes in the future.