Andromeda, the Milky Way's neighboring spiral galaxy, may be a cosmic cannibal that has been growing by chowing down on smaller galaxies.
Astronomers reached this conclusion by observing a globular cluster of stars called the Dulais Structure, named after the Welsh term for "black stream," which now dwells in Andromeda but seems to have originated from outside the galaxy. The Dulais Structure is illuminated by star clusters that orbit unlike any other clusters in Andromeda, which means the stream of stars could represent the "leftovers" from a galaxy-size meal in Andromeda's past.
The findings support the idea that galactic growth happens violently and sporadically, when large galaxies eat their smaller counterparts.
"We've come to realize over the last few decades that galaxies grow by eating smaller systems — so little galaxies fall in, they get eaten — it's galactic cannibalism," Geraint Lewis, an astrophysicist at the University of Sydney and lead author of the new research, said in a statement. "A few years ago, we discovered that in the far outskirts of Andromeda, there was a sign in the objects orbiting it that the galaxy hadn't been grazing, but it had eaten large quantities in two distinct epochs."
If this new model of Andromeda's growth is correct, Lewis said, the next question is, exactly what the spiral galaxy has eaten of these smaller galaxies.
"That then leads to the next question of, well, what was actually consumed?" he said. "Because it doesn't look like it was just one thing; it looks like it's been a collection of things that are all being slowly torn apart."
Lewis and his colleagues discovered that Andromeda shows signs of two major feeding events in its past: One meal happened sometime in the past 5 billion years, and the other was between 8 billion and 10 billion years ago.
As the 13.8 billion-year-old galaxy has aged, it has expanded, meaning that during these mealtimes, matter in the universe would have been more densely concentrated than it is today. Since then, galaxies like Andromeda have formed and grown, turning the universe from a homogeneous desert of matter to the feature-filled cosmos we see today — although how precisely that played out remains unclear.
"We know that the universe was featureless at its birth in the Big Bang, and today it's full of galaxies," Lewis said. "Were those galaxies born fully formed, or have they grown?"
Studying galaxies like Andromeda helps astrophysicists better understand how galaxies evolve, because seeing them from the outside offers a more complete picture than observing the Milky Way from our position within it, the researchers said.
Is our galaxy also a cosmic cannibal?
While there is some evidence that our galaxy has been merging with and even swallowing other galaxies, it isn't yet settled our Milky Way galaxy, which is similar in size and shape to Andromeda, has also engaged in bouts of galactic cannibalism to facilitate its own growth. However, the clear picture of feeding events and growth spurts in Andromeda does show that this is occurring in the local universe.
"What this new result does is provide a clearer picture of how our local universe has come together," Lewis said. "It is telling us that at least in one of the large galaxies, that there has been this sporadic feeding of small galaxies. What we want to know is, has the Milky Way done the same, or is it different?"
Lewis and his team now aim to pinpoint when the Andromeda feeding events occurred. This information will be important in perfecting models of galactic evolution. To determine the dates of the galaxy's meals, the researchers must understand the distances at play in Andromeda between its 'homegrown' features and the remains of other, consumed galaxies. This will allow the team to recreate Andromeda's history in three dimensions.
"That will then allow us to work out orbits, where things are going, and then we can start to run the clock backward and see if we can get this coherent picture of when things fell in," Lewis said.
The first indications of the Dulais Structure's origin as "leftovers" came in the work of two students: Tim Adams, of the University of Sydney, and Yuan Li, of the University of Auckland.
"When they come to you and say, 'I keep getting this signal, and it's a bit weird,' that's when it gets very exciting," Lewis said. "It's opened a new door in terms of our understanding. But exactly what it's telling us — I think we still have to work that one out."
The team's research has been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society, and a preprint version is available via arXiv.
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Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.