Saturday, December 4, 2021

There is a strange shape in the middle of Andromeda, and astronomers have finally found out why

In the center of the Andromeda Galaxy, about 2.5 million light-years from us and in the vicinity of the Milky Way, is a mysterious star cluster. For decades, this has caused astronomers to wrinkle their eyebrows and stroke their chins.

However, new research into how galaxies – and supermassive black holes at their centers – can collide with each other may offer an explanation for this cluster. It looks like it could have been caused by a gravitational “push”, something similar to the recoil of a shotgun, but on a cosmic scale.

This latest study suggests that the impact will be powerful enough to create an elongated mass of millions of stars – technically known as an eccentric nuclear disk – instead of the symmetrical star cluster that is usually found in the center of a galaxy like Andromeda.

“When galaxies merge, their supermassive black holes come together and ultimately become a single black hole,” said astrophysicist Tatsuya Akiba of the University of Colorado at Boulder. “We wanted to know: what are the consequences of this?”

To find out, the team ran computer simulations of collisions between supermassive black holes. The resulting force would be enough to pull the orbits of stars near the galactic center into an elongated shape, as seen on Andromeda, according to calculations.

When galaxies collide, their respective black holes are believed to orbit each other, picking up speed before eventually colliding together. As you can imagine, this generates a huge amount of energy released in the pulses of gravitational waves – we are talking about literal ripples in the fabric of space and time.

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Although the waves created by these mergers do not directly affect the stars in the galaxy, they can affect their position. Based on models assembled by the researchers, the remaining supermassive black hole could receive a powerful push in one direction. If the jolt is not too strong (or too weak), it can drag off a bunch of nearby stars with it.

“These gravitational waves will blow the momentum away from the remaining black hole, and you get a gun-like recoil,” Akiba says.

Orbit of stars around a supermassive black hole before (l) and after (r) gravitational “impact”. (Stephen Burroughs / Gila)

Some supermassive black holes can move so fast that they leave their home galaxy altogether. As for those who don’t, it looks like they can still leave star clusters deformed in an eccentric nuclear disk.

It is believed that there are up to 2 trillion galaxies in the universe, and not all of them follow the same pattern. The researchers say their work may help explain some of the differences in star clusters that appear outside of Andromeda and the Milky Way.

The next step for this particular type of analysis is to scale it up compared to hundreds stars used in these computer models to millions stars, and perhaps apply the same modeling approach to different types of galaxies.

“This idea – if you’re in orbit around a central object and the object suddenly flies off – can be scaled down to explore many different systems,” says astrophysicist Anne-Marie Madigan of the University of Colorado at Boulder.

Research published in Letters to the astrophysical journal

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