Andromeda contains the remnants of a recent “Feeding Event”

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

“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 was not grazing, but that it had eaten large amounts in two different eras,” said Professor Geraint Lewis . from the University of Sydney.

Lewis is the lead author of a new paper titled “Chemodynamic Substructure in the M31 Inner Halo Globular Clusters: Further Evidence for a Recent Accretion Event.” The Monthly Notices of the Royal Astronomical Society will publish the paper and it is currently available on the prepress site arxiv.org.

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“We’ve come to realize over the past few decades that galaxies grow by eating up smaller systems — so few galaxies fall in, they get eaten — it’s galactic cannibalism.”

Professor Geraint Lewis, University of Sydney

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

Globular clusters are the focus of this research. They are older associations of lower metallicity stars. 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 common in a galaxy’s halo, while their counterparts, open clusters, are found in the galactic disks.

This image shows one of the oldest globular clusters known, called Messier 15. It's about 35,000 light-years away in the constellation Pegasus (the Winged Horse), and it's about 12 billion years old.  Messier 15 is one of the densest globular clusters known, with most of its mass concentrated in its core.  Image credit: NASA, ESA.
This image shows one of the oldest globular clusters known, Messier 15. It lies about 35,000 light-years away in the constellation Pegasus (the Winged Horse) and is about 12 billion years old. Messier 15 is one of the densest globular clusters known, with most of its mass concentrated in its core. Image credit: NASA, ESA.

The researchers behind this work identified a population of globular clusters 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. The globular clusters have a lower metallicity than most stars in the same region, meaning they came 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 called the collection of globulars the Dulai structure, which means black stream in Welsh.

The Dulais structure is likely a group of between 10 and 20 globular clusters misaligned with Andromeda’s rotation. But they’re not the only misaligned group of globular clusters. The Dulais structure is evidence that Andromeda fed on a group of globular clusters sometime in the past 5 billion years. The other group is a subpopulation of globular clusters that is evidence of a second feeding event between 8 and 10 billion years ago.

This figure from the study shows the metallicity and rotational axes of Andromeda's globular clusters.  The gray ellipse shows Andromeda's rotation, and the blue shows the spherical objects with higher metallicity.  The higher metallicity blue spherical spheres, which must be younger, are closely aligned with Andromeda's rotation.  Green and red are spherical objects with lower metallicity.  They are offset from Andromeda's rotation, so it is likely that they were absorbed from elsewhere.  This is the feature the researchers call the Dulais structure.  Image credit: Lewis et al. 2022
This figure from the study shows the metallicity and rotational axes of Andromeda’s globular clusters. The gray ellipse shows the rotation of Andromeda and the blue indicates the higher metallicity globular clusters. The blue spherical spheres with higher metallicity, which must be younger, are more closely aligned with Andromeda’s rotation. Green and red are spherical objects with lower metallicity. They are offset from Andromeda’s rotation, so they were probably absorbed from elsewhere. This is the feature the researchers call the Dulais structure. Image credit: Lewis et al. 2022

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

This illustration shows the motion of the Dulais structure in the Andromeda galaxy.  Image Credit: Geraint Lewis
This illustration shows the motion of the Dulais structure in the Andromeda galaxy. Image credit: Geraint Lewis

To Lewis and his co-authors, the Dulais structure resembles the leftovers from a messy meal. It is a dark stream with vibrant clusters of stars. It is further evidence that massive galaxies are merging to produce giant displays across the Universe and that larger galaxies are consuming smaller globular clusters in a kind of galactic cannibalism.

“That then leads to the next question: what was actually consumed? Because it doesn’t look like it was just one thing, it looks like it’s been a collection of things that are all slowly being torn apart,” Professor Lewis said. “We’ve come to realize over the past few decades that galaxies grow by eating up smaller systems — so few galaxies fall in, they get eaten — it’s galactic cannibalism.”

When these feeding events occurred, matter in the universe was more tightly concentrated. Ten billion years ago there may have been more of these events in the universe. That’s one of the reasons why astronomers yearn for increasingly powerful telescopes like the James Webb. They can see the light of old galaxies and look further back in time.

“We know that the Universe was featureless at its birth in the Big Bang, and today it is full of galaxies. Were those galaxies born fully formed or did they grow?” Professor Lewis said.

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

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

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

This image shows the Milky Way, according to data from ESA's Gaia space probe.  The Milky Way consumes the Sagittarius dwarf galaxy, a collection of four globular clusters.  Image credit: by ESA/Gaia/DPAC, CC BY-SA 3.0 igo, https://commons.wikimedia.org/w/index.php?curid=77752828
This image shows the Milky Way, according to data from ESA’s Gaia space probe. The Milky Way consumes the Sagittarius dwarf galaxy, a collection of four globular clusters. Image credit: by ESA/Gaia/DPAC, CC BY-SA 3.0 igo, https://commons.wikimedia.org/w/index.php?curid=77752828

As it stands, Lewis and the other researchers only have a two-dimensional historical view of the Dulais structure. The dimensions are speed and chemistry. Finding the distances of all these objects yields a 3rd dimension, which will fill in the history of the globular clusters and how Andromeda consumed them. Lewis isn’t quite sure at this point that we can call them spherical, and he won’t be until there is more data. Hence the name “Dulais structure”.

“That then allows us to work out trajectories, where things are going, and then we can run the clock back and see if we can get this cohesive picture of when things fell in,” he said.

“We couldn’t call it an object like a galaxy, because we don’t really know if the signature we’re seeing is from one large object disrupting or seven smaller objects disrupting. So we call it a structure rather than it a particular galaxy.”

There is clearly something going on with the Dulais structure and the Andromeda galaxy. But true to his scientific training, Lewis is cautious about firm conclusions at this stage.

“It’s opened a new door in terms of our understanding,” Lewis said in a press release. “But what exactly it tells us, I think we still have to work that out.”

This illustration shows the globular clusters of the Dulais structure scattered throughout Andromeda Image Credit: Geraint Lewis
This illustration shows the globular clusters of the Dulais structure scattered throughout Andromeda Image Credit: Geraint Lewis

The authors state their case clearly in their paper. Intriguingly, 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 the globular clusters in the Dulais structure is that they follow the accretion of a substantial precursor (about 1011 solar masses) in Andromeda’s halo over the last few billion years, which may have occurred as part of a larger group incursion.

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