A string of 13 dwarf galaxies have been found in orbit around the massive galaxy Andromeda, spread across a flat plane more than one million light years wide and only 30,000 light years thick. They appear to be moving in synchronicity with each other. The discovery suggests that conventional ideas regarding the formation of galaxies are missing something fundamental.
Pointing Us Toward Something Profound
“It’s a very unusual, unexpected configuration,” said astrophysicist Dr. Julio Navarro, at the University of Victoria in 2016. “It’s so unexpected that we don’t know yet what it’s telling us. The fact that it is there at all is pointing us toward something profound. Somehow, they have a plane-like structure similar to a solar system, but with a completely different origin and we don’t know what that origin is.”
Understanding how and why the dwarf galaxies form the ring around Andromeda is expected to offer new information on the formation of all galaxies. The dwarf galaxies range in size from 10 million to 100 million stars and twelve of the 13 are on one side of the orbital plane, as if they are held by a string being swung from Andromeda.
Like some pre-existing structure has been sucked in by Andromeda
“This looks like they are all moving together and they all know where to go, like some pre-existing structure has been sucked in by Andromeda,” Navarro said.
“Orbiting Together in an Immense Plane”
“When we looked at the dwarf galaxies surrounding Andromeda, we expected to find them buzzing around randomly, like angry bees around a hive,” said galactic archaeologist Geraint F. Lewis at the Sydney Institute for Astronomy at the University of Sydney in 2018. “Instead, we’ve found that half of Andromeda’s satellites are orbiting together in an immense plane, which is more than a million light years in diameter but only 30 000 light years thick. These dwarf galaxies have formed a ring around Andromeda. This was completely unexpected – the chance of this happening randomly is next to nothing. It really is just weird.”
For several decades, astronomers have used computer models to predict how dwarf galaxies should orbit large galaxies, and every time they found that dwarfs should be scattered randomly over the sky. Never, in these simulations, did they see dwarfs arranged in a plane like that observed around Andromeda.
“Now that we’ve found that the majority of these dwarf galaxies orbit in a disc around the giant galaxy Andromeda, it looks like there must be something about how these galaxies formed or subsequently evolved that has led them to trace out this peculiar coherent structure,” said Professor Lewis. “Dwarf galaxies are the most numerous galaxy type in the universe, so understanding why and how they form this disc around the giant galaxy is expected to shed new light on the formation of galaxies of all masses.”
“There has not been any ground shift on the consensus about the origin of these structures,” Navarro told The Daily Galaxy. “It is clear that the distribution of satellite galaxies around the Milky Way and Andromeda (and others) is markedly anisotropic, with a subset of those satellites on a 2D structure suggestive of a ‘plane’.
“Cosmological simulations are able to reproduce, in general, the anisotropy of the satellite distribution,” he adds, “but it is harder to reproduce the planar distributions. Whether this is a general issue that highlights a fundamental problem with our cosmological understanding, or just a peculiarity that arises from focusing on one or two galaxies (doesn’t any single member of a population look distinctly unique in some respect is not completely clear.”
Image credit: NASA