There's a Weird Shape in The Middle of Andromeda, And Astronomers Finally Know Why

A mysteriously-shaped cluster of stars is located at the center the Andromeda Galaxy. It's approximately 2.5 million light years away from the Milky Way. This star cluster has been making astronomers furrow their eyebrows and stroke their heads for decades.
New research on how supermassive black hole centers and galaxies can collide together could provide an explanation. This cluster could be due to a gravitational "kick", something similar to the recoil from a shotgun, but on a cosmic level.

The latest study shows that the kick could be strong enough to create an extended mass of millions stars, technically called an eccentric nuclear disc. This is in contrast to the symmetric star clusters that would normally be found at the center of a galaxy such as Andromeda.

Tatsuya Akiba from the University of Colorado Boulder says that when galaxies combine, their supermassive dark holes will come together and eventually form a single black hole. "We wanted to find out: What are the consequences?"

The team conducted computer simulations of supermassive dark hole collisions to find out. According to calculations, the force generated would be strong enough to stretch the orbits of stars close to a galactic center.

It is believed that galaxies collide and their black holes spin around one another, picking up speed, before finally colliding. You can see, this generates a lot of energy. It is released in gravitational waves pulses. These are literally ripples in space and time.

Although these waves don't directly affect the galaxy's stars, they can have an impact on their position. The researchers have created models that show how the supermassive black holes can be given a powerful jolt in one direction. It can pull a lot of stars nearby if the shove isn’t too strong or too weak.

Akiba says that gravitational waves can carry momentum away from the black hole and create a recoil similar to a gun's recoil.

The orbit of stars around supermassive black holes before (l), and after (r), a gravitational "kick". (Steven Burrows/JILA)

Some supermassive dark holes are capable of traveling so fast that they can escape their home galaxy. Those that do not have supermassive black holes can still leave star clusters warped by an eccentric nuclear disk.

The Universe is home to up to 2 trillion galaxies. However, they are not all the same. Researchers believe their research could explain some of the star cluster diversity that appears outside of Andromeda or the Milky Way.

Next, this type of analysis will be scaled up from the hundreds to millions of stars in the computer models. And perhaps the same simulation approach can be applied to galaxies with different types.

Ann-Marie Madigan, an astrophysicist at the University of Colorado Boulder, says that this idea of being in orbit around a central object when it suddenly flies off can also be scaled down to study many different systems.

Astrophysical Journal Letters published the research.