spaceSpace and Physics

Andromeda’s Arms Caused By Galactic Collision

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Lisa Winter

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clockJun 4 2014, 18:16 UTC
1117 Andromeda’s Arms Caused By Galactic Collision
NASA/JPL-Caltech/K. Gordon

Andromeda (M31), our closest galactic neighbor at 2.5 million light years away, is slated to collide with the Milky Way in about 4 billion years to form one huge elliptical galaxy. However, this won’t be the first collision in Andromeda’s history. New computer simulations have suggested that a dwarf galaxy with a supermassive black hole at the center collided with Andromeda, causing the large galaxy to form its spiral arms, which look like rings from Earth’s perspective. Understanding how this happens could help astronomers understand spiral galaxies, including the Milky Way. The announcement comes from researchers at Harvard led by Avi Loeb. The results will appear in the journal Astrophysical Journal Letters and is available in preprint on

Andromeda is a bit of an optical illusion, as it doesn’t have clearly defined arms like other spiral galaxies. Instead, it sort of looks like it has concentric rings, like Saturn. This odd appearance has led many astronomers to speculate that Andromeda used to be a disc galaxy, but a collision with dwarf galaxy M32 essentially created a ripple gravitational effect, creating the appearance of rings or arms.


This isn’t the first time computer simulations have attempted to explain Andromeda’s appearance with a collision. A team from the University of Witwatersrand in South Africa developed a simulation that was published in Nature in 2006. A key difference between that study and the one completed by Loeb’s team was that the 2006 paper assumed M32 crashed very near the center of Andromeda. They also concluded that Andromeda has true rings, not spiral arms. However, Loeb’s team feels it is fairly unlikely that M32 would have hit the bullseye like that, when the odds would suggest it would hit anywhere else.

Loeb’s simulation puts the collision in the outer regions of the disc. They believe that over the course of two billion years, M32 crashed into Andromeda and continued to pass through, though the rippling effect tore up the disk and began to form arms. It then took M32 about 900 million years to slow down, reverse direction, and come back where it will tear through Andromeda again. They also determined that Andromeda does have true spiral arms, but they just look like rings from Earth’s perspective because of the angle and the fact that they are tightly wrapped arms.

Additionally, the simulation revealed new details of how M32 may have behaved during the collision. Traditionally, astronomers believed that gravitational forces would have pulled away a dwarf galaxy’s outer stars, gas, and dark matter, which would reduce its size. However, the simulation suggests that M32 was already so compact, Andromeda wasn’t able to pull off that outer envelope of material. This must mean that M32’s compact elliptical nature was formed by some other means.


Despite Andromeda being our closest neighbor, there isn’t a whole lot known about it. As observational data continues to be collected, more simulations will have to run in order to simulate situations that can actually be tested. 


[Hat tip: Katia Moskvitch, Nature News]

spaceSpace and Physics
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  • spiral galaxy,

  • Andromeda,

  • dwarf galaxy,

  • galactic collisions,

  • M32,

  • M31