A highly detailed survey of the nearby Andromeda Galaxy, which as a nearby spiral galaxy is something of an observational proxy for our own Milky Way, shows the remnants of smaller galaxies that our neighbor appears to have cannibalized.

The Pan-Andromeda Archaeological Survey is not yet complete but is already producing scientific results such as those in this week's Nature. (Scientific American is part of the Nature Publishing Group.) The study's high-resolution look at Andromeda provides further confirmation of the prevailing theory of galactic growth—that today's giant galaxies fed on smaller companions to reach their present size.

The survey, which relies on observations made at the Canada–France–Hawaii Telescope atop Mauna Kea in Hawaii, is led by astrophysicist Alan McConnachie, a research fellow at the National Research Council Canada's Herzberg Institute of Astrophysics in Victoria, British Columbia.

Astronomers enjoy studying Andromeda in part because of its proximity—at 2.5 million light-years away it is close enough to us that individual stars can be resolved within the galaxy. And Andromeda can be seen in its entirety, unlike the Milky Way, in which we are deeply embedded. "If you're inside a city, and you're trying to figure out what that city looks like, it's much harder," McConnachie says. What is more, he adds, Andromeda appears to be a fairly representative spiral galaxy, based on comparisons with others in the universe at large, so unraveling the history of its formation may help astronomers understand how galactic processes work in general.

The dominant cosmological theory holds that galaxies grow through a process of accretion—by cannibalizing smaller galaxies and incorporating their mass in an ever-growing amalgamation. In agreement with similar observations of the Milky Way, the Andromeda survey has found a sort of archaeological record of this process—the fossilized remains of devoured galaxies.

"We have this idea of how we think galaxies form, and that's to do with the merging together of smaller galaxies," McConnachie says. "And if that's true, then when we look over the vast area around a galaxy, we should actually be able to see remnants of the formation process." Those remnants, which McConnachie calls "the partially digested remains of these dwarf galaxies," take the form of large, diffuse streams of stars, former galactic groupings that have been pulled apart by the larger galaxy's gravitational pull.

The Andromeda survey is turning up several of those remnants, just as predicted by the accretion theory. "When I look at this picture, it really makes me happy," says James Bullock, a cosmologist at the University of California, Irvine, who has worked on what galactic formation residues should look like. "This is exactly the kind of thing that we would expect—these streams of stars, this pretty clear evidence that you've had all these past accretion events."

Bullock notes that this is not the first time such evidence has been found—in fact, some of McConnachie's co-authors published evidence for galactic cannibalism in Andromeda in 2001. But each increasingly detailed look at the galaxy provides more examples and, hence, a more convincing case. "You're never that confident in your theory until you really see it proved by observation," Bullock says. "So it's not humdrum; it's exciting in that sense."

The new survey also indicates that Andromeda's growth continues apace, and on a massive scale to boot. The panoramic portrait reveals a possible past interaction between the galaxy and a smaller neighbor, Triangulum, as evidenced by a newly discovered stellar appendage on Triangulum that extends in Andromeda's direction. That feature, based on the research team's simulations, could be explained by a close encounter between the two galaxies in the past few billion years that ripped a section of stars from Triangulum.

"It actually looks as if it is being destroyed by Andromeda," McConnachie says, which would be a surprise given how much distance separates the galaxies. If this assessment is correct, McConnachie says, then in a few billion years' time—around the same time Andromeda and the Milky Way are expected to come together—Andromeda will finish off Triangulum, creating an even larger supergalaxy.