Colliding Cannibalistic Galaxies Emit a Devastating Gravitational 'Kick'

And researchers think this might explain Andromeda's distorted shape.

Colliding Cannibalistic Galaxies Emit a Devastating Gravitational 'Kick'
Graphic showing the before (left) and after (right) of a gravitational 'kick'. Steven Burrows/JILA

The universe is full of cannibals. Cannibalistic galaxies, that is. And it turns out that, when one galaxy collides with, or feeds on, another, the supermassive black holes at the center of both galaxies emit a colossal gravitational "kick" that can be compared to the recoil of a powerful firearm. 

A new paper from researchers at the University of Colorado Boulder suggests that this cosmic kick is so powerful, it can shift the orbits of millions of stars and distort the shape of an entire galaxy. The research, published in The Astrophysical Journal Letters, might provide a solution to a decades-long mystery regarding the shape of our neighboring galaxy, Andromeda. 

The mystery of the Andromeda galaxy's oval center

The Andromeda galaxy was first observed in detail in the 1970s, followed by extensive observations in the 1990s with the Hubble Space Telescope. Hubble's observations revealed an elongated, oval mass at Andromeda's center, where astronomers expected to find a circular cluster of stars surrounding the galaxy's supermassive black hole.

As Andromeda is thought to have formed when two supermassive black holes collided billions of years ago, the UC Boulder researchers used computer simulations to model what occurs during such an event. They found that the force generated during the collision of two supermassive black holes could bend and pull the orbits of stars near a galactic center. They believe this may have caused Andromeda's oval shape. The team also says its finding might help to explain the great variety of differently-shaped galaxies that have been observed in the universe, many of which are less symmetrical than the Milky Way.

Colliding Cannibalistic Galaxies Emit a Devastating Gravitational 'Kick'
The Andromeda galaxy. Source: Wikimedia Commons

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


The UC Boulder researchers' simulations show that, when galaxies collide, the black holes at their centers may begin to spin around each other at enormous speeds until they eventually slam into each other. When this happens they release massive "gravitational waves" that are, essentially, 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, like the recoil of a gun," Akiba explained.

A supermassive black hole traveling at millions of miles per hour

Interestingly, the researchers believe that it isn't the recoil itself that directly throws surrounding stars into a wonky orbit. Instead, their models suggest that the gravitational 'kick' throws the remaining supermassive black hole traveling through space at speeds of millions of miles per hour. The resulting gravitational influence of this shifting supermassive black hole is what then throws the orbit of millions of stars out of whack, creating an asymmetrical shape like the one seen at the center of Andromeda.    


Next, the researchers want to improve their simulations and directly compare their computer results to real data regarding Andromeda. In a 2019 study, astronomers revealed that the Milky Way will likely be eaten by Andromeda in approximately 4 billion years. The latest research from UC Boulder leaves us wondering what shape the resulting merger will take and whether the Milky Way will cause a similar distortion hinting at its cosmic history.

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