A decades-old mystery about the Andromeda galaxy might have just been solved. The galaxy's central supermassive black hole is orbited by star clusters in a peculiar formation, and researchers now believe that a collision with another supermassive black hole caused this.

As reported in The Astrophysical Journal Letters, researchers conducted simulations of the collision between two supermassive black holes, finding that the clusters of stars orbiting these two objects end up in a peculiar elongated distribution.

"When scientists first looked at Andromeda, they were expecting to see a supermassive black hole surrounded by a relatively symmetric cluster of stars," co-author professor Ann-Marie Madigan, a fellow of JILA, a joint research institute between CU Boulder and the National Institute of Standards and Technology (NIST), said in a statement. "Instead, they found this huge, elongated mass."

"When galaxies merge, their supermassive black holes are going to come together and eventually become a single black hole," added lead author Tatsuya Akiba, a graduate student at CU boulder. "We wanted to know: What are the consequences of that?"

A collision between galaxies takes billions of years. Over that period, the supermassive black holes at their original center would move closer and closer together as a new galaxy is born.

The swirling motion of gas and stars pushes the supermassive black holes to spiral in towards one another, and the huge gravity of supermassive black holes orbiting each other also produces gravitational waves that take away momentum, creating a recoil. This recoil can make the supermassive black hole fast.

"If you're a supermassive black hole, and you start moving at thousands of kilometers per second, you can actually escape the galaxy you're living in," Madigan said.

However, when the merged black hole doesn’t escape, it can mess with the orbit of the clusters in its proximity. The team found orbital patterns that reminded them of the particular configuration of the central region of the Andromeda galaxy.

The finding is a good working theory for what happened to Andromeda. Galaxies merge often over the lifetime of the universe (and Andromeda and our own galaxy the Milky Way are destined to collide). The team is now interested in creating more sophisticated simulations with more stars to see if they can better reproduce what we observed at the core of Andromeda and even explain, in this way, the past experiences of more distant galaxies.