A string of 13 dwarf galaxies in orbit around the massive galaxy Andromeda –remnants of the population of primordial structures that coalesced to form giant galaxies like the Milky Way–are spread across a flat plane more than one million light years wide and only 30,000 light years thick –a distance so vast that they have yet to complete a single orbit. The 2016 discovery suggests that conventional ideas regarding the formation of galaxies are missing something fundamental.
Somehow, they have a plane-like structure similar to a solar system, but with a completely different origin.”
“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. Twelve of the 13 dwarf galaxies — they range in size from 10 million to 100 million stars — are on one side of the orbital plane, as if they are held by a string being swung from 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.
This was completely unexpected – the chance of this happening randomly is next to nothing. It really is just weird.”
“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 synthetic universes, 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.”
The Last Word –“Do We Have Gravity Wrong?
“The origin of the planes is still a bit of a mystery and there are continuing arguments around their formation and lifetime,” Lewis wrote in an email to The Daily Galaxy. “There have been some recent claims that *maybe* they fit with the standard cosmological model, but many are not convinced with the argument. Another suggestion is that planes are formed in gravity interactions ( known as tidal dwarfs – here’s a recent paper on these) but if they are, then the dwarfs would have no dark matter and would quickly dissolve. And some have suggested that the way to make them survive is by requiring that we have gravity wrong and need to modify it. This, of course, is a radical shift from our current understanding, but if we can’t find how planes form and survive within our favored cosmological model, we’ll be faced with having to consider crazier possibilities!”
NASA image at top of the page shows the Andromeda galaxy. Also known as Messier 31 or M31, located about 2.4 million light-years away from Earth, it is the largest galaxy in the Local Group, which also contains our own Milky Way Galaxy and more than 50 other galaxies
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Editor, Jackie Faherty, astrophysicist, Senior Scientist with AMNH. Jackie was formerly a NASA Hubble Fellow at the Carnegie Institution for Science. Aside from a love of scientific research, she is a passionate educator and can often be found giving public lectures in the Hayden Planetarium. Her research team has won multiple grants from NASA, NSF, and the Heising Simons foundation to support projects focused on characterising planet-like objects. She has also co-founded the popular citizen science project entitled Backyard Worlds: Planet 9 which invites the general public to help scan the solar neighbourhood for previously missed cold worlds. A Google Scholar, Faherty has over 100 peer reviewed articles in astrophysical journals and has been an invited speaker at universities and conferences across the globe. Jackie received the 2020 Vera Rubin Early Career Prize from the American Astronomical Society, an award that recognises scientists who have made an impact in the field of dynamical astronomy and the 2021 Robert H Goddard Award for science accomplishments.