Andromeda Galaxy Had a Much More Violent History Than Our Milky Way

Astronomers have taken a closer look at the nearby Andromeda galaxy and have made some startling discoveries. They've found striking differences from our own Milky Way, suggesting a much more violent history of mergers with smaller galaxies.

In this latest study, the researchers combined data from two large surveys of stars in Andromeda, one conducted by the W.M. Keck Observatory and the other using the Hubble Space Telescope. The surveys measured more than 10,000 individual bright stars in Andromeda, creating high-resolution imaging at six different wavelengths.

"In the Andromeda galaxy, we have the unique combination of a global yet detailed view of a galaxy similar to our own," said Puragra Guhathakurta, one of the researchers, in a news release. "We have lots of detail in our own Milky Way, but not the global, external perspective."

The high resolution of the Hubble images allowed the scientists to separate stars form one another in the crowded disk of Andromeda. This analysis revealed a clear trend related to stellar age, with the youngest stars showing relatively ordered rotation motion around the center of the Andromeda galaxy while older stars displayed much more disordered motion.

So what caused this particular formation? The researchers considered different scenarios of galactic disk formation and evolution that could account for their observations. One of these scenarios involved the gradual disturbance of a well-ordered disk of stars as a result of mergers with small satellite galaxies. The alternate scenario involved the formation of the stellar disk from an initially thick, clumpy disk of gas that gradually settled.

What was more interesting is that by comparing Andromeda to the Milky Way, the researchers found substantial differences suggesting that Andromeda had a much more violent accretion history in the recent past. It's possible that Andromeda grew by cannibalizing smaller satellite galaxies. Cosmologists predict that 70 percent of disks the size of Andromeda's and the Milky Way's should have interacted with at least one sizable satellite in the last 10,000 years. The Milky Way's disk is far too orderly for that to have happened. However, Andromeda's disk fits this scenario.

"In this context, the motion of the stars of Andromeda's disk is more normal, and the Milky Way may simply be an outlier with an unusually quiescent accretion history," said Guhathakurta.

The findings were presented at a meeting of the American Astronomical Society.

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