A fascinating discovery about dwarf galaxies orbiting the Andromeda galaxy suggests that conventional ideas regarding the formation of galaxies like our own Milky Way are missing something fundamental. A string of 13 dwarf galaxies in orbit around the massive galaxy Andromeda are spread across a flat plane more than one million light years wide and only 30,000 light years thick, moving in synchronicity with one another, according to University of Victoria astronomer Julio Navarro, one of the co-authors of an article on the phenomenon in the latest edition of the journal Nature. The dwarfs are spread across a distance so vast that they have yet to complete a single orbit.
The behavior of Andromeda's dwarfs is so extreme from the usual chaotic orbits of galaxies around each other that the the researchers believe they have revealed a huge hole in science’s understanding of galaxy formation. Computer models show that the dwarf galaxies should orbit independently, almost randomly. But the structure of the synchronous galaxies orbiting Andromeda is much more like a mature solar system.
In a paper published in the journal Nature, an international team of astronomers described the discovery that almost half of the 30 dwarf galaxies orbiting Andromeda do so in an enormous plane more than a million light years in diameter, but only 30,000 light years thick. The findings defied scientists’ expectation—based on two decades of computer modeling—that satellite galaxies would orbit in independent, seemingly random patterns. Instead, many of these dwarf galaxies seem to share a common orbit, an observation that currently has no explanation.
“It’s a very unusual, unexpected configuration,” says UVic astrophysicist Dr. Julio Navarro, a co-author of the paper. “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,” Navarro said. 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.
The paper is based on data collected for a project led by UVic professor Alan McConnachie of the Herzberg Institute of Astrophysics in Saanich.