Andromeda contains the remains of a recent “feeding event”.

There is a growing body of evidence that galaxies grow exponentially 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 major galaxy to the Milky Way, and a new study shows that our neighbor has consumed other galaxies in two different eras.

“A few years ago, we discovered that on the far outskirts of Andromeda, there was a sign in the objects orbiting it that the galaxy was not grazing, but ate large amounts in two different epochs,” said Professor Geraint Lewis. from the University of Sydney.

Lewis is the lead author of a new paper, “Dynamic Chemical Ultrastructure in M31’s Inner Halo-Global Clusters: Further Evidence for a Recent Accretion Event.” Monthly Notices of the Royal Astronomical Society will publish the paper, and it is currently available in preprint at arxiv.org.

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“We’ve realized over the past few decades that galaxies grow by eating smaller systems – so a few galaxies fall in, and they get eaten – it’s galactic cannibalism.”

Professor Geraint Lewis, University of Sydney

“What this new finding does is provide a clearer picture of how our local universe holds together — it tells us that at least in one of the large galaxies, there has been this intermittent feeding of smaller galaxies,” Lewis said in a paper.

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

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

The researchers behind this work have identified a group of globules in Andromeda’s inner halo that all contain the same mineral. Metallicity refers to the elemental composition of stars, with elements heavier than hydrogen and helium being referred to as metallics in astronomy. The globules have a lower metallicity than most stars in the same region, which means they came from somewhere else, not from Andromeda itself. It also means that they are older because there were fewer heavy elements in the early universe than there are now. Lewis named the group of globular spheres the Dulai Structure, which means black stream in Welsh.

Dulais’ structure is likely to be a collection of between 10 and 20 deflector spheres with Andromeda spinning. But they aren’t the only skewed group of spheroids. Dulais’ structure is evidence that Andromeda ate a group of globular globules sometime in the last 5 billion years. The other group is a subgroup of spheroids that is evidence of a second feeding event 8 to 10 billion years ago.

This figure shows the mineralogy and rotational axes of globular clusters in Andromeda.  The gray ellipse shows the rotation of Andromeda, and the blue the higher metallic globules.  The higher blue globules, which should be younger, are closely aligned with the rotation of Andromeda.  Green and red are low metallic spheroids.  They're compensated for Andromeda's spin, so it's possible that they'll be sucked in from somewhere else.  This is a feature that researchers call the structure of the lys.  Image credit: Lewis et al.  2022
This figure shows the mineralogy and rotational axes of globular clusters in Andromeda. The gray ellipse shows the rotation of Andromeda, and the blue indicates the higher metallic globules. The higher blue globules, which should be younger, are more in line with Andromeda’s rotation. Green and red are low metallic spheroids. They’re compensated for Andromeda’s spin, so it’s possible that they’ll be sucked in from somewhere else. This is a feature that researchers call the structure of the lys. Image credit: Lewis et al. 2022

According to Lewis and his co-authors, globular clusters have a lower mineral content and are also kinetically distinct from other clusters in the same area. The Andromeda Galaxy rotates in one direction, and Dulais’ structure moves differently.

This illustration depicts the motion of the Dulais structure within the Andromeda Galaxy. Image Credit: Geraint Lewis
This illustration depicts the motion of the Dulais structure within the Andromeda Galaxy. Image credit: Geraint Lewis

To Lewis and his co-authors, Dulays’ structure looks like leftovers from a messy meal. It is a dark stream containing vibrant star clusters. It is further evidence that massive galaxies are merging to produce giant displays throughout the universe and that larger galaxies are consuming small spheres in a kind of galactic cannibalism.

This then brings us to the next question, what is actually consumed? Because it didn’t seem like it was just one thing, Professor Lewis said, it seemed like it was a bunch of things that were all slowly being torn apart. “We’ve realized over the past few decades that galaxies grow by eating smaller systems – so a few galaxies fall in, and they get eaten – it’s galactic cannibalism.”

When these feeding events occurred, the matter in the universe was more tightly concentrated. Ten billion years ago, there might have been more such events throughout the universe. This is one of the reasons why astronomers want more and more powerful telescopes like James Webb. They can see the light from the old galaxies and look back in time.

“We know that the universe was featureless when it was born in 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 Milky Way galaxy. We all will. It’s hard to do that with notes because we’re so embedded in it. But Andromeda offers an opportunity to study the evolution of galaxies from an outsider’s perspective, and researchers like Lewis and his colleagues are taking full advantage. As a spiral galaxy similar to the Milky Way, some of what astronomers learn about galaxy mergers from Andromeda could apply to our galaxy as well.

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

“What we want to know is, did the Milky Way do the same thing, or are they different? Both have interesting consequences for the overall picture of how galaxies form,” said Professor Lewis. “We want, on some level, to devise a more accurate clock to tell us when these events happen because that’s something we need to factor into our models of how galaxies evolve.”

This image shows the Milky Way according to data from the European Space Agency's Gaia spacecraft.  The Sagittarius dwarf galaxy, a group of four globular clusters, is consumed by the Milky Way.  Image credit: by ESA/Gaia/DPAC, CC BY-SA 3.0 igo, https://commons.wikimedia.org/w/index.
This image shows the Milky Way according to data from the European Space Agency’s Gaia spacecraft. The Sagittarius dwarf galaxy, a group of four globular clusters, is consumed by the Milky Way. Image credit: by ESA/Gaia/DPAC, CC BY-SA 3.0 igo, https://commons.wikimedia.org/w/index.

As it is, Lewis and other researchers only have a two-dimensional historical view of the Dulles structure. Dimensions are speed and chemistry. Finding the distances of all of these objects will provide a third dimension, which will fill in the history of globulars and how Andromeda consumed them. Lewis isn’t entirely sure we can call them spherical at this point, and won’t be until more data is available. Hence the name “Haikal al-Dulais”.

“That will then allow us to work on the orbits, where things are going, and then we can start to turn back the clock and see if we can get this coherent picture of when things fall,” he said.

“We can’t call it an object like galaxy because we actually don’t know whether the signature we’re seeing is caused by one large object disrupting or seven smaller objects disrupting. That’s why we kind of refer to it as a structure rather than a specific galaxy.”

There’s clearly something going on with Dulais’ structure and the Andromeda Galaxy. True to his scientific training, however, Lewis is cautious about firm conclusions at this point.

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

This illustration depicts globular clusters of Dulais structure scattered across Andromeda. Image Credit: Geraint Lewis
This illustration depicts globular clusters of Dulais structure scattered across Andromeda. Image Credit: Geraint Lewis

The authors state their case clearly in their paper. “Interestingly, the orbital axis of this Dulais structure aligns closely with that of a smaller accretion event recently identified using a subset of globular clusters in the outer halo of Andromeda, and this strongly suggests a causal relationship between the two,” the authors summarize in their paper.

“If this association is confirmed, the natural explanation for the kinetics of the globular clusters in the Dulais structure is that they follow the accumulation of a large precursor (about 1011 solar masses) in the Andromeda halo during the past few billion years, which may have occurred as part of a larger cluster infallibility. “

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