Graphic showing the orbits of stars around supermassive black holes before, after and after a gravitational "kick". Credit: Steven Burrows/JILA
The gravitational "kick" that occurs when two galaxies collide is similar to the sound of a shotgun's recoil. According to new research from CU Boulder, this kick can cause millions of stars to be placed in unnatural orbits.
The research was published in The Astrophysical journal Letters on Oct. 29 and helps to solve decades-old mysteries surrounding the strangely-shaped clusters of stars that make up the Andromeda Galaxy. This research could also be used to help scientists better understand how galaxies form by feeding on one another.
Ann-Marie Madigan, a fellow of JILA (a joint research institute between CU Boulder and NIST), said that scientists expected to see a supermassive dark hole surrounded by a cluster of stars. They found this massive, extended mass instead.
She and her colleagues now believe they have an answer.
Scientists launched balloons into the atmosphere of Earth in the 1970s to observe ultraviolet light from Andromeda, the nearest galaxy to the Milky Way. In the 1990s, the Hubble Space Telescope continued to make observations and discovered a surprising fact: Andromeda looks like our galaxy's giant spiral. The area near the spiral's center is rich in stars, but it doesn't appear like it should. Instead, these stars' orbits take on an odd and ovalish shape similar to Silly Putty.
Madigan, an assistant professor in astrophysics, stated that no one knows why. Scientists refer to the pattern as an "eccentric nucleus disk".
The team used computer simulations to see what happens when supermassive black hole collide. Andromeda was likely formed from a similar merger billions years ago. The team calculated that the force created by such a merger could bend or pull the orbits stars close to a galactic centre, creating the distinctive elongated pattern.
Tatsuya Akiba is a graduate student studying astrophysics and the lead author of this study. "We wanted to find out: What are the implications of that?"
Space and time can be bent
He said that the team's findings helped to reveal some of those forces that could be driving the diversity among the universe's estimated two trillion galaxies today. Some of these look like the spiral-shaped Milky Way, others more like footballs or irregularly blobs.
These mergers could play a significant role in shaping these masses: Akiba stated that when galaxies collide with each other, black holes at their centers may start to spin around one another, moving faster than ever until they eventually collide. They release massive pulses of "gravitational wave," which are literally ripples in space and time.
Akiba stated that gravitational waves would carry momentum away from any remaining black holes, and you will get a recoil similar to a gun's recoil.
Madigan and he wanted to find out what such a recoil could cause to stars within 1 parsec (roughly 19 trillion miles) of a star's center. Andromeda can be seen from Earth with the naked eye. It spans tens to thousands of parsecs.
It can get pretty wild.
Galactic recoil
Computers were used to create models of fake galactic centres containing hundreds stars. The central black hole was then kicked to simulate gravitational waves.
Madigan explained that the gravitational waves created by such a catastrophic collision will not directly affect stars in a galaxy. The recoil will send the supermassive black holes back through space at speeds of millions of miles an hour. This is not bad considering that they have masses millions to billions times larger than Earth's sun.
Madigan stated, "If you are a supermassive dark hole and you move at thousands of kilometers per seconds, you can escape the galaxy that you live in."
The team discovered that black holes can pull on the orbits around stars, which could cause them to expand. This result ends up looking very similar to the shape scientists see in the center of Andromeda.
Madigan and Akiba stated that they would like to expand their simulations in order to directly compare their results with the real-life galaxy core, which contains many more stars. Their findings could also be used by scientists to help them understand unusual events around other objects in our universe, like planets orbiting mysterious bodies called nucleon stars.
Madigan stated that this idea, if you are in orbit around a central object but suddenly it flies offcan easily be scaled down to study lots of other systems.
Continue exploring Supermassive blackholes can eat on one star each year
Tatsuya Akiba and colleagues, on the Formation of an Eccentric Nuclear Disk Following the Gravitational Recoil kick of a Supermassive Black Hole. The Astrophysical Journal Letters (2021). Information from Astrophysical Journal Letters Tatsuya Akiba and colleagues, On the Formation an Eccentric Nuclear Disk Following the Gravitational Recoil kick of a Supermassive Dark Hole, (2021). DOI: 10.3847/2041-8213/ac30d9
