Andromeda. A dark stream sheds new light on the life of galaxies

An international team of scientists led by a University of Sydney astrophysicist has discovered evidence that the Andromeda galaxy is a cannibal that grows through colossal intermittent feasts.

The research, which is available on the arXiv preprint server and will be published in the Monthly Notices of the Royal Astronomical Society, is based, in part, on the unexpected findings of two honors students.

“A few years ago, we discovered that just outside Andromeda there was a signal in the objects orbiting it that the galaxy had not been grazing, but had eaten large amounts at two separate epochs,” said lead author Prof. Geraint Lewis from the University of Sydney.

“What this new result does is provide a clearer picture of how our local universe has come together: it tells us that in at least one of the large galaxies, there has been this sporadic feeding of small galaxies.”

The research findings build on the discovery of a structure of stars, known as globular clusters, in Andromeda that originated outside the galaxy. Professor Lewis called this the Dulais Structure, derived from the Welsh for black flux.

The Dulais Structure represents the remnants of a colossal feeding event in the “recent” past, a dark stream illuminated by star clusters that orbit Andromeda like no other. It provides evidence that galaxies grow by “eating” smaller systems, and the findings are at odds with a calmer picture of galactic growth.

“That leads to the next question of, well, what was actually consumed? Because it doesn’t seem like it was just one thing, it seems like it’s been a collection of things that are slowly being torn apart,” the professor said. Luis. “We’ve realized in recent decades that galaxies grow by eating smaller systems, so small galaxies fall in and eat them, it’s galactic cannibalism.”

Andromeda has the signatures of two major food events. The approximate time scales indicate that the “recent” feast took place sometime in the last 5 billion years, while the oldest feeding was closer to 8 to 10 billion years ago. The universe itself is 13.8 billion years old, which means that the two separate events may have taken place while matter in the universe was closer together and more densely concentrated.

“We know that the universe was featureless at its birth in the Big Bang, and today it is filled with galaxies. Were these galaxies born fully formed or have they grown?” said Professor Lewis.

Astrophysicists like Professor Lewis are studying Andromeda to better understand how our own Milky Way has evolved. The vantage point from Earth makes it difficult to see our galaxy because we are sitting inside it, which obscures observations, but the distance from Andromeda allows scientists the advantage of a “bird’s eye view.”

It’s unclear how the Milky Way has been fed, but a picture is emerging in Andromeda with a clear signature: big feasts and growth spurts. Since the Milky Way is a similarly sized spiral galaxy, the research may be painting a picture of what our galaxy has done to reach its enormous size.

Next steps

“What we want to know is whether the Milky Way has done the same or is different. Both have interesting consequences for the big picture of how galaxies form,” said Professor Lewis. “We want, at some level, to create a more accurate clock that tells us when these events occurred because that’s something we need to include in our models of how galaxies evolve.”