The Andromeda Galaxy is, officially, a monster. Astronomers studying our next-door neighbor have discovered that the huge spiral galaxy has quite the cannibalistic past. On two separate occasions over the last 10 billion years, Andromeda has gobbled up smaller galaxies, but the way in which it devoured them has astronomers puzzled.
Our galaxy, but Andromeda is on a whole different level.
"We've known for about 10 to 15 years that Andromeda has had a much more violent past, in terms of swallowing smaller galaxies, than the Milky Way has," said Dougal Mackey, a researcher at Australian National University and first author on the new study.
The research, conducted by Mackey and an international team of collaborators, was published in the journal Nature on Oct. 2 and reconstructs the history of the Andromeda galaxy in greater detail than ever before. After examining 77 globular clusters -- dense knots of around a million stars each -- orbiting Andromeda and measuring their velocities, the team stumbled upon something pretty surprising.
The best explanation for their data suggests Andromeda cannibalized a galaxy (or multiple galaxies) at least twice over the last 10 billion years. One of these feeding frenzies occurred recently, but the other is much more ancient. Rather than gradually absorbing stars and clusters over time, Andromeda has been "binge-eating in distinct sessions," according to Geraint Lewis, an astronomer with Sydney University and co-author on the study.
It gets a bit weirder, though. The way that Andromeda's two binge sessions occurred has intrigued the researchers because it appears the galaxy's meals came from two different directions. The older clusters are spinning around Andromeda in a way similar to how rings spin around Saturn. But the other cluster moves around Andromeda perpendicular to the aging cluster.
"If that is the case, then there appear to be specific directions from which galaxies 'prefer' to arrive," said Mackey.
That phenomenon could point to a galaxy formation role for the network of filaments and voids forming the basis of our universe, the so-called "cosmic web."
The web is "a sponge-like distribution of matter, with dense knots and empty voids, and filaments and sheets in between," said Lewis. You might look at the universe and think all the stars were just randomly dropped into place, as if an almighty god just scattered its marbles, but the cosmic web shows how the universe is richly connected. The hypothesis then, is that this web helped feed Andromeda its meals and that's why we see clusters rotating around it at 90 degrees.
"We know that matter flows out of voids and along the filaments and into galaxies, which might suggest that Andromeda has been getting food from defined directions from the cosmic web."
Lewis says the team aren't quite sure that hypothesis holds true but it is an intriguing notion.
The team will now analyze the globular clusters used in the research in greater detail, hoping to pull apart the most recent Andromeda feed and determine exactly what it ate. Was it one galaxy? Two? Many small dwarf galaxies? That's what they hope to find out.
Mackey also says they hope to "weigh" Andromeda.
"At the moment we are not even certain whether the Milky Way or Andromeda is the more massive system," he said. "It's very difficult to accurately weigh galaxies … but the clusters might give us a way forward here."
The monster living next door is eventually going to swallow up our home galaxy, too.
Of course, there's no need to panic. The Milky Way's demise by galactic cannibalism is slated to occur some 4 billion years from now. Hopefully, astronomers will come to some conclusions about Andromeda's cannibalistic tendencies and its weight before then.