Andromeda contains the remnants of a recent “feeding event”

There is growing evidence that galaxies grow by merging with other galaxies. Telescopes like Hubble have captured dozens of interacting galaxies, including well-known galaxies like Arp 248. The Andromeda Galaxy is the closest large galaxy to the Milky Way, and a new study shows our neighbor has consumed other galaxies at two distinct epochs.

“A few years ago we discovered that on the outskirts of Andromeda there was a sign in the objects orbiting it that the galaxy had not grazed, but had eaten large amounts at two distinct eras,” Professor Geraint Lewis said. from the University of Sydney.

Lewis is lead author of a new paper titled “Chemodynamic substructure in globular clusters of the M31 inner halo: further evidence for a recent accretionary event.” The Royal Astronomical Society Monthly Notices will publish the article, and it is currently available on the prepress site arxiv.org.

“We’ve realized over the past few decades that galaxies grow by eating smaller systems – if few galaxies fall, they’re eaten – that’s galactic cannibalism.”

Professor Geraint Lewis, University of Sydney

“What this new result does is provide a clearer picture of how our local universe came together – it tells us that at least in one of the larger galaxies there was this sporadic feeding small galaxies,” Lewis said in a press release. Release.

Globular clusters are at the center of this research. These are associations of older stars that have lower metallicity. There are at least 150 in the Milky Way, probably more. They play a role in galactic evolution, but the role is not clearly understood. Globulars, as they are called, are more prevalent in a galaxy’s halo, while their counterparts, open clusters, are found in galactic disks.

The researchers behind this work have identified a population of globulars in Andromeda’s inner halo that all have the same metallicity. Metallicity refers to the elemental composition of stars, with elements heavier than hydrogen and helium called metals in astronomy. Globulars have lower metallicity than most stars in the same region, which means they come from somewhere else, not Andromeda itself. It also means they are older because there were fewer heavy elements in the early Universe than there are now. Lewis named the collection of globulars the Dulai structure, which means black stream in Welsh.

The Dulais structure is probably a group of 10 to 20 globulars misaligned with the Andromeda rotation. But they are not the only group of misaligned globulars. The Dulais structure is evidence that Andromeda fed on a group of globulars for the past 5 billion years. The other group is a subpopulation of globulars that bears witness to a second feeding event 8–10 billion years ago.

According to Lewis and his co-authors, globular clusters have lower metallicity and are also kinematically distinct from other clusters in the same region. The Andromeda galaxy rotates in one direction and the structure of Dulais moves differently.

For Lewis and his co-authors, the Dulais structure resembles the remains of a messy meal. It is a dark stream containing clusters of vibrant stars. This is further evidence that massive galaxies merge to produce gigantic screens throughout the Universe and that larger galaxies consume smaller globulars in a type of galactic cannibalism.

“That then leads to the next question, well, what was actually consumed? Because it doesn’t appear to be one thing, it appears to be a set of slowly tearing things apart,” Professor Lewis said. “We have realized over the last few decades that galaxies grow by eating smaller systems – if few galaxies fall, they are eaten – that is galactic cannibalism. “

When these feeding events occurred, matter in the Universe was more highly concentrated. Ten billion years ago there may have been more of these events in the Universe. This is one of the reasons why astronomers want increasingly powerful telescopes like the James Webb. They can see the light of ancient galaxies and look further back in time.

“We know that the Universe was flat when it was born during the Big Bang, and today it is full of galaxies. Were these galaxies born fully formed or did they grow? said Professor Lewis.

Astronomers would like to know the history of our own galaxy, the Milky Way. We all would. It’s hard to do through observations because we’re embedded in them. But Andromeda presents an opportunity to study galaxy evolution from an external perspective, and researchers like Lewis and his colleagues are taking full advantage of it. As a spiral galaxy similar to the Milky Way, some of what astronomers learn about Andromeda galaxy mergers may also apply to our galaxy.

But astronomers still have work to do before they can draw any conclusions about the Milky Way. Or about mergers and consumption in general. The goal is a more detailed timeline of galactic evolution throughout the Universe.

“What we want to know is if the Milky Way did the same thing or is it different? Both of these have interesting consequences for the overall picture of galaxy formation,” Professor Lewis said. “We want, on some level, to come up with a more accurate clock to tell us when these events happened because that’s something we need to include in our models of galaxy evolution.”

As things stand, Lewis and the other researchers only have a two-dimensional historical view of the Dulais structure. Dimensions are speed and chemistry. Finding the distances of all these objects will provide a 3rd dimension, which will fill in the story of the globulars and how Andromeda consumed them. Lewis isn’t absolutely sure we can call them globular at this point, and he won’t be until there’s more data. Hence the name “Dulais Structure”.

“That will then allow us to figure out the orbits, where things are going, and then we can start rolling the clock back and see if we can get this consistent picture of when things fell,” he said.

“We couldn’t name it as an object like a galaxy because we don’t know if the signature we see is from one large disturbing object or seven smaller disturbing objects. That’s why we kind of call it a structure rather than a particular galaxy.

Obviously something is going on with the Dulais structure and the Andromeda galaxy. But true to his scientific background, Lewis is cautious about making definitive conclusions at this point.

“It opened a new door in terms of understanding,” Lewis said in a press release. “But exactly what he’s telling us, I think we still have to figure out.”

The authors clearly state their case in their article. “Curiously, the orbital axis of this Dulais structure is closely aligned with that of the younger accretion event recently identified using a subpopulation of globular clusters in Andromeda’s outer halo, and this strongly suggests a causal relationship between the two,” the authors summarize in their paper.

“If this connection is confirmed, a natural explanation for the kinematics of globular clusters in the Dulais structure is that they trace the accretion of a substantial progenitor (about 10¹¹ solar masses) in the Andromeda halo over the past billion years, which may have occurred as part of a larger group fall.

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