Binary Black Holes

Of all the mysteries of the universe, black holes are one of the most interesting—discussed in all seriousness in universities and with great levity at dinner parties. Professor Liu Fukun of the Department of Astronomy and the Kavli Institute for Astronomy and Astrophysics at Peking University has taken the lead on the discovery of two supermassive black holes orbiting each other in a far off, quiet galaxy.

Most galaxies, including our own beloved Milky Way, have supermassive black holes at the center, but fi nding two super-massive black holes, destructive forces almost beyond measure, in orbit is rare. It has been said that this fi nd is the “smoking gun”for galactic collision, the moment when two galaxies intertwine and merge together. Our own galaxy is due to collide with the Andromeda Galaxy in about four billion years, and we might, perhaps, also host two supermassive black holes.

Science is in the business of prediction, the ability to calculate future events based on previous observation. In this, the discovery of these two black holes validated Liu’s previous work in tidal disruption, enabling him to spot one of the universe’s most mysterious masses via careful calculation of data. The discovery was an international effort, with contributions from the Max-Planck Institute and the European space observatory XMM-Newton, but no one quite knew what they were looking at, then simply considered data for one supermassive black hole with a bit of a “wobble”. It was Liu’s observations on tidal disruption that fi nally put a face to this galaxy’s odd pulses of X-rays.

Liu has put forward an interesting model for the existence of these supermassive black holes. It depicts a primary black hole with ten million solar masses (one solar mass equal to the mass of earth’s sun) with a secondary black hole orbiting at around one million solar masses. He was also able to calculate that the distance between the two is approximately the distance of our solar system. For the next two million years, these black holes will continue their dance in the unceremoniously named galaxy SDSS J120136.02+300305.5; at which point they will merge into one.

This discovery has pulled binary black holes out of the realm of the theoretical and into the observable. Previous candidates for binary supermassive black holes have been posited, but the calmness of this J120136 galaxy is what made its discovery so elegant. Liu’s team noticed that the loud X-rays from this quiet galaxy faded out for 21 days, only to return and then follow a predictable fading pattern; it was an orbit. Normally, supermassive black holes are assumed to be in loud, active galaxies, but it was perhaps the relative peace of this galaxy that expedited the discovery—subtracting the noise of so many stars and gas clouds being ripped apart by black holes ten million times the mass of our sun.

As with so much else in the world of physics, this discovery paves the way to more questions. What will it look like when these two black holes merge? Some think a duck bill shape will emit from the looming collision of event horizons. Can black hole merger recoil send the black hole shooting off into the galaxy at an incredible speed, gobbling up everything in its path? NASA scientists think they’ve seen a black hole doing just that in CID-42. So much more remains to be discovered concerning these binary black holes, but, at the very least, Liu and his team have given us all stable ground on which to watch one of the most destructive forces in the known universe dance with a partner its equal.