There is a growing body of evidence that galaxies grow by merging with other galaxies.
Telescopes like Hubble have captured dozens of interacting galaxies, including some known as arp 248.
The Andromeda galaxy is the closest large galaxy to the Milky Way, and a new study shows that our neighbor has consumed other galaxies at two separate epochs.
“A few years ago, we discovered that on the outskirts of Andromeda there was a signal in the objects that orbited it that the galaxy had not been grazing, but had eaten large amounts at two different times.” said Geraint Lewis from the University of Sydney.
Lewis is lead author of a new paper titled “Chemodynamic substructure in the globular clusters of the inner halo M31: further evidence for a recent accretionary event.” The Monthly Notices of the Royal Astronomical Society will publish the article, and it is currently available on the prepress site arxiv.org.
“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,” Lewis said. said in a press release.
globular clusters are at the center of this investigation.
They are older associations of 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 known, are most prevalent in the halo of a galaxy, while their counterparts, open clusters, are found in galactic disks.
The researchers behind this work 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 referred to as metals in astronomy.
Globulars have a lower metallicity than most stars in the same region, which means they come from somewhere else, not Andromeda itself.
It also means that they are older, since 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 between 10 and 20 globulars misaligned with Andromeda’s rotation. But they are not the only group of misaligned globulars.
The Dulais Structure is evidence that Andromeda fed on a group of globulars sometime in the last 5 billion years. The other group is a subpopulation of globulars that is evidence of a second feeding event between 8 and 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 Dulais Structure moves in a different way.
To Lewis and his co-authors, the Dulais Structure looks like the leftovers of a messy meal. It is a dark stream that contains vibrant star clusters. It’s further evidence that massive galaxies are merging to produce giant screens throughout the Universe, with larger galaxies consuming smaller globulars in a kind of galactic cannibalism.
“That leads to the next question of, well, 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 slowly being torn apart.” 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.”
When these feeding events occurred, the matter in the Universe was more concentrated. Ten billion years ago, there may have been more of these events in the entire Universe. That’s one reason astronomers want ever more powerful telescopes like the James Webb. They can see light from ancient galaxies and look further back in time.
Astronomers would like to know the history of our own galaxy, the Milky Way. We all would. That is difficult to do through observations because we are immersed in it.
But Andromeda presents an opportunity to study galaxy evolution from an outside perspective, and researchers like Lewis and his colleagues are making the most 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 have more 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 whether the Milky Way has done the same or differently. Both have interesting consequences for the overall picture of how galaxies form,” Lewis said. said.
“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.”
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 will provide a third dimension, completing 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 more data is available. Hence the name “Dulais Structure”.
“That will allow us to calculate the orbits, where things are going, and then we can start to turn the clock back and see if we can get this consistent picture of when things fell,” he said. said.
“We couldn’t name it as an object like a galaxy because we don’t actually know if the signature we see is from one large object being disrupted or seven smaller objects being disrupted. That’s why we refer to it as a structure in place of that.” being 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 any firm conclusions at this stage.
“It has opened a new door in terms of our understanding,” Lewis said in a press release. “But exactly what it’s telling us, I think we still have to figure it out.”
The authors make their case clearly in their article. “Interestingly, the orbital axis of this Dulais Structure is closely aligned with that of the recently identified youngest accretionary event using a subpopulation of globular clusters in Andromeda’s outer halo, and this strongly suggests a causal relationship between the two,” the authors write. summarize in your article.
“If this connection is confirmed, a natural explanation for the kinematics of globular clusters in the Dulais Structure is that they track the accumulation of a substantial progenitor (about 10eleven solar masses) in the Andromedan halo during the last few billion years, which may have occurred as part of a larger falling group.”