The Intergalactic Battle of Colliding Black Holes

Nazifa Younus

Contributing Writer


Two billion years from now our galaxy is in for a shock. With every hour that passes, the Milky Way galaxy gets half a million kilometers closer to another sizable spiral galaxy called Andromeda, and it is only a matter of time before we collide. Yet, the picture is far from complete. Lying at the center of our galaxy is a giant black hole more than three million times as massive as the Sun. The black hole at the heart of Andromeda is believed to be ten times that size. 

Most, if not all, galaxies have a supermassive black hole at their centers. Everyone thought that these hungry behemoths sat at the heart of their parent galaxies, vacuuming up gas clouds and ripped-apart stars. Similar to the Milky Way, Andromeda is shaped like a giant spiral. When scientists first saw Andromeda, they expected to see a supermassive black hole surrounded by relatively symmetrical clusters in its center. Instead, they found a vast, elongated mass. The orbits of these stars had a strange oval shape. Scientists call this pattern an “eccentric nuclear disc.” A new study led by University of Colorado Boulder has solved a decades-old mystery surrounding a strangely-shaped cluster of stars at the heart of the Andromeda Galaxy. What is the reason behind this deformation?

In the 1970s, scientists launched balloons high into Earth’s atmosphere in order to take a closer look at the ultraviolet light of Andromeda. The Hubble Space Telescope followed up on those initial observations in the 1990s and delivered a surprising finding: the area rich in stars near that spiral’s center doesn’t look as scientists had predicted; the orbits of these stars take on an odd, ovalish shape. Tatsuya Akiba, a lead author of the study and a graduate student in astrophysics, created computer simulations to track what happens when two supermassive black holes go crashing together. This is an area of interest because our own Milky Way black hole and Andromeda’s black hole will probably collide when the galaxies themselves collide.

Their calculations suggest that the force generated by such a merger could bend and pull the orbits of stars near a galactic center; based on team calculations, the forces generated by such unions can turn or draw the star’s trajectory. When two galaxies collide, the black holes at their cores are thought to go into orbit around each other. Gravity pulls them ever closer, so the black holes spiral together until they merge, releasing gravitational waves all the while. The final moments before two black holes collide, when gravity is strongest, have remained obscure.

By far, the most exciting consequence of a black hole merger, though, is the “kick” the merged object can receive. The size of the kick depends crucially on the spin because unequal spins make the merger asymmetric, and that produces asymmetric gravitational waves. These act like rocket exhaust, pushing the black hole in the opposite direction; in the consolidation of galaxies with relatively small black holes, the kick may be only a few hundred kilometers per second, so the merged object may be booted only as far as the outer regions of its parent galaxy before falling back to the center. 

Since the merged object may very well take its super-hot disc of swirling matter and  with it. Thus, it will appear as a very bright, compact object called a quasar, displaced from the center of the galaxy. Mergers may play an essential role in shaping these masses of stars. In the process of collision, they release vast pulses of gravitational waves or literal ripples in the fabric of space and time. Those gravitational waves will carry momentum away from the remaining black hole, and you get a recoil similar to the recoiling of a gun after shooting. Scientists wanted to know what such a recoil could do for a star within a parsec, or about 19 trillion miles of the galactic center. 

Visible to the naked eye from Earth, Andromeda stretches tens of thousands of parsecs from end to end. Using computer models, scientists built models of fake galactic centers containing hundreds of stars. They then kicked the central black hole to simulate the recoil from gravitational waves. Akiba and his team’s findings help to reveal some of the forces that may be driving the diversity of the estimated two trillion galaxies in the universe today. The galactic center creates that distinct elongated pattern of the galaxy as a whole. This explained gravitational waves and their overall effect. 

What is produced by this kind of catastrophic collision does not directly affect the galaxy’s stars. Ann-Marie Madigan, a fellow of JILA, a joint research institute with UC Boulder, said, “the gravitational waves produced by this kind of disastrous collision won’t affect the stars in a galaxy directly. But the recoil will throw the remaining supermassive black hole back through space––at speeds that can reach millions of miles per hour. At that speed, black holes can escape the galaxy they are in. When black holes don’t escape the galaxy they are in, the team discovers they might pull on the orbits of the stars right around them, causing those orbits to stretch out. Madigan and Akiba said they would like to expand the simulation so that computer results can be compared directly with the core of the actual galaxy. They also expressed that their findings might help scientists understand anomalous events around other objects in the universe, such as planets orbiting a neutron star.