Our nearest galactic neighbor engaged in ‘cannibalism’ over and over again, study finds: ScienceAlert

There is increasing evidence that galaxies become large by merging with other galaxies.

Telescopes such as Hubble have captured dozens of interacting galaxies, including well-known ones like Arp 248.

The Andromeda Galaxy is the closest large galaxy to the Milky Way, and a new study shows that our neighbor swallowed up other galaxies in two different epochs.

“A few years ago, we discovered that on the far outskirts of Andromeda, there is a sign in the objects that orbit it that the galaxy has not grazed, but eaten large amounts in two different eras,” said Geraint Lewis from the University of Sydney.

Lewis is the lead author of a new paper titled “Hemodynamic substructure in the inner halo globular cluster M31: Further evidence for a recent accretion event.“The Monthly Notices of the Royal Astronomical Society will publish the article and it is currently available at arxiv.org preprint site.

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

Globular clusters are the focus of this study.

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 known, are more common in a galaxy’s halo, while their counterparts, open clusters, are found in galactic disks.

The researchers behind this work identified a population of globules 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 being called metals in astronomy.

Globulars have a lower metallicity than most stars in the same region, meaning they came from somewhere else rather than Andromeda itself.

This also means they are older, as there were fewer heavy elements in the early universe than there are now.

Lewis named the collection of globular structures the Dulai Structure, which means black stream in Welsh.

The Duhle structure is likely a group of between 10 and 20 globular bodies that are not aligned with Andromeda’s rotation. But they are not the only misaligned group of globulars.

The Duhle 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 orbs 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 metallicities and are also kinematically different from other clusters in the same region. The Andromeda galaxy rotates in one direction and the Dulles structure moves in another direction.

To Lewis and his co-authors, Duhle’s structure looks like the remains of a messy meal. It is a dark stream containing living star clusters. This is further evidence that massive galaxies are merging to produce giant displays throughout the Universe, and that larger galaxies are swallowing smaller globular bodies in a kind of galactic cannibalism.

“This then leads to the next question, well, what was actually consumed? Because it doesn’t look like it was just one thing, it looks like it was a collection of things all slowly tearing apart.” said Lewis.

“Over the last few decades, we’ve realized that galaxies grow by eating smaller systems — so small galaxies fall in, get eaten — that’s galactic cannibalism.”

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

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

Astronomers would like to know the history of our own Milky Way galaxy. We all would. This is difficult to do through observations because we are embedded in it.

But Andromeda provides an opportunity to study galaxy evolution from an outside 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 are learning about galaxy mergers from Andromeda can be applied to our galaxy.

But astronomers have more work to do before they can draw conclusions about the Milky Way. Or at all about mergers and acquisitions. The goal is a more detailed chronology of galactic evolution throughout the universe.

“What we want to know is, did the Milky Way do the same thing, or did it do it differently?” Both have interesting implications for the overall picture of how galaxies form,” Lewis said.

“We want at some level to come up with a more accurate clock that tells us when these events happened, because that’s one thing we need to incorporate into our models of how galaxies evolve.”

As it stands, Lewis and other researchers have only a two-dimensional historical view of the Dulet structure.

The dimensions are speed and chemistry. Finding the distances of all these objects will provide a third dimension that will fill out the story of the globulars and how Andromeda swallowed them.

Lewis isn’t entirely sure we can call them globular at this stage, and won’t be until more data is available. Hence the name “Dule Structure”.

“This will then allow us to calculate 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 are happening,” he said.

“We wouldn’t be able to call it a galaxy-like object because we don’t really know if the signature we’re seeing is from one large object destroying or seven smaller objects destroying. That’s why we kind of call it a structure rather than a specific galaxy.”

Apparently something is going on with the Dule structure and the Andromeda galaxy. But true to his scientific background, Lewis is cautious about drawing firm conclusions at this stage.

“It 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 out.”

The authors make their case clear in their paper. “Interestingly, the orbital axis of this Duhle structure is closely related to that of the younger accretion event recently identified using a subpopulation of globular clusters in Andromeda’s outer halo, strongly suggesting 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 Duhle structure is that they trace the accretion of a significant progenitor (about 10¹¹ solar masses) in Andromeda’s halo over the past few billion years, which may have occurred as part of a larger cluster infall.”

This article was originally published by The universe today. Read on original article.