Ep. 634: Milky Way’s Mergers & Acquisitions

The Milky Way is a vast grand spiral today, but how did it get this way? Astronomers are starting to unravel the history of our galaxy, revealing the ancient collisions with dwarf galaxies, and how they came together to build the Milky Way.

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Show Notes

Latex allergy tweets (Dr. Pamela’s Twitter)

Gaia (ESA)

Arp’s Catalog Of Peculiar Galaxies


Giant Galaxies from the Universe’s Childhood Challenge Cosmic Origin Stories (Scientific American)

Dwarf Galaxy (Swinburne University)

NASA’s Hubble Shows Milky Way is Destined for Head-On Collision (NASA)

The Local Group is our galactic neighborhood (EarthSky)


The Gaia Sausage: The Major Collision that Changed the Milky Way Galaxy (Carnegie Mellon University)


Stellar Streams are Revealing Their Secrets (Sky & Telescope)

What are Magellanic Clouds? (Universe Today)

What is a globular cluster? (EarthSky)

VIDEO: What are Globular Clusters? Relics of the Early Universe (Fraser Cain)

Star Clusters (Harvard)

VIDEO: Building an Artificial Magnetosphere with Elena D’Onghia (Fraser Cain)

PDF: Multiple Stellar Populations in Globular Clusters (arxiv)

The Disk of the Milky Way is Warped Because it Already Collided With Another Galaxy (Universe Today)

Galaxies Have Been Found With no Dark Matter at all (Universe Today)

UCI scientists discover how galaxies can exist without dark matter (University of California, Irvine)

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Transcriptions provided by GMR Transcription Services

Fraser:                         AstronomyCast Episode 634, Milky Way Mergers and Acquisitions. Welcome to AstronomyCast, your weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hey, Pamela, how you doing?

Dr. Gay:                      I’m having an allergy day, so if I look a little bedraggled, it’s because I have a latex allergy. I tweeted all about this yesterday, and then today, made a hair elastic mistake, so the red eyes that are tearing up have nothing to do with anything in the world and everything to do with one lone misbegotten hair elastic.

Fraser:                         Yeah. I also have red eyes that are tearing up today, and mine is just seasonal allergies, and a little congestion in my nose, so I sound a little congested, but welcome to springtime in the Northern Hemisphere. The Milky Way is a vast, grand spiral today, but how did it get this way? Astronomers are starting to unravel the history of our galaxy, revealing the ancient collisions with dwarf galaxies and how they came together to build the modern Milky Way. All right, Pamela. How can we do this episode and not just have me ranting and enthusiastically praising the Gaia space mission? Is that possible?

Dr. Gay:                      Well, we can talk about the fact that mergers aren’t just in the Milky Way, and how –

Fraser:                         They happen in other places too?

Dr. Gay:                      – and the Arp catalogue of irregular galaxies, and –

Fraser:                         All right, all right.

Dr. Gay:                      – interactions leave dark-matter-less galaxies.

Fraser:                         Yeah, there’s gonna be a lot of time spent praising Gaia.

Dr. Gay:                      Gaia is amazing.

Fraser:                         Yeah, just be prepared. Just be emotionally equipped to handle the effuse praise for the Gaia mission. All right. So, where did astronomers think galaxies came from?

Dr. Gay:                      So, originally, they didn’t. For a long time, we thought that the galaxies we were seeing out there were just nebulae. They’re not. Once we started to figure out that there are these other island universes out there that are full of stars and have myriad different structures, two different ideas came about, and at that point in the history of astronomy, we believed it was an either/or, not an and, and the explanations were that it was either what we call top-down or bottom-up, where either a whole bunch of mass came together, merged, formed stars, and a galaxy was born out of whole cloth like Athena coming out of the skull of Zeus.

Fraser:                         Right, just one big collapse, broke it up into little pieces, stars formed, galaxy done. Bigger or smaller, just depending on how much material they had to work with.

Dr. Gay:                      Exactly. Then, the other idea was you only form small things, you only form star clusters, dwarf galaxies, that’s it. Little things. And, over time, those come together, merge, merge some more, and bigger and bigger systems are able to form. Now we understand it’s a both/both thing that appears to happen, but that’s one of the cool things about modern astronomy, is we’re realizing all these things where people got into raging arguments of “No, it is only top-down,” “No, it is only bottom-up.” It’s both, people. It’s both.

Fraser:                         Yeah. And, we are a little ahead of the gun in that one of James Webb’s primary jobs is to be able to peer out into the early universe and ideally see these processes happening directly, to actually image the smaller structures as they came together. So, a year from now, we will be able to give you a much better answer about how this works. But, thanks to the Gaia mission and also our observations of other galaxies, we still see these merger processes, these collisions happening both in the Milky Way and around us.

Dr. Gay:                      Yes. And, what I’m loving is impatient astronomers got super creative in finding ways to do the science JWST is designed to do using much more complicated processes, but they still got there, and were able to image that at the very beginnings of the universe, the first galaxies we’re seeing have among them giant galaxies. And yet, as we look around our current universe at the things nearest to us, including ourselves, we see little galaxies constantly merging into bigger systems, midsized galaxies, small galaxies all colliding. It is galaxy hits galaxy all the way down.

Fraser:                         It is interesting that it continues to be this bit of a paradox. Every time Hubble finds a more distant galaxy, the gist of the article is “This galaxy is far more mature and complete than astronomers ever thought they should see,” and then, at the same time, there are dozens of dwarf galaxies buzzing around the Milky Way, all of which are doomed. A collision is due for us with Andromeda that we see galaxy collisions and mergers all around us, and so, I don’t know, it sort of feels obvious now that it is both. They got big quick, but also, they got finished off with lots of other smaller mergers.

Dr. Gay:                      Yeah. And, this idea that things evolve differently in different environments is actually part of what I was studying with my dissertation. It’s a project that is still getting worked on today, where we are understanding that the largest things, whether it be the largest stars or the largest individual galaxies, they just collapsed first, gravity collapsed them down quickly, and in these larger gravitational fields, you end up with the environment getting pulled in more effectively, creating higher-dense clusters and these higher-dense environments. You have more chances for things to fly past each other, to directly collide together.

This evolves things faster, both stripping out dust, triggering star formation, leading to these massive, fully evolved clusters. But, when you look at small groups of galaxies, like the one we live in, our own local group, you don’t have as much mass, and things are moving slower, things aren’t interacting as much because there’s just not as much to interact, and this allows star formation to just slowly chew forward without these massive bursts of star formation. It allows the smaller galaxies time to get consumed. It allows the larger galaxies time to evolve, and then fall into one another.

Fraser:                         So, with the launch of the Gaia spacecraft – and, this is the European Space Agency’s finest astrometry machine, which is amazing, which is giving us the location of 2 billion stars, the motion of them. We’re learning a tremendous amount just about how the various stars are moving around inside the Milky Way. And, how has this cast a lot of light on how the Milky Way grew?

Dr. Gay:                      I don’t know if you even meant the pun that it was brilliant. “Cast light”?

Fraser:                         Okay, right.

Dr. Gay:                      What’s amazing is it’s not just the positions. They’re also getting high-caliber photometry and hints at the chemistry of these stars, and this allows them to get at these groups of stars moving on similar orbits with similar ages and similar compositions must have all come in through one giant galaxy-eats-galaxy event or one small one, and we’re starting to get direct evidence of things that we had only modeled before, the idea that our Milky Way has a variety of different components.

There’s the main high-density part of the disc, there’s the thick disc, which is lower-density, where stars tend to have more tilty orbits, for lack of the better phrase. There’s that spheroid that goes around the entire system. And, we said that the thick disc was likely due to collisions with other systems, and now we can look and say, “Yes, this specific set of stars going in this specific set of orbits in the thick disc. Yes, those came from a merger.”

Fraser:                         You can see some of the pictures that are associated with these research papers, where they’ll have almost a 3D image of the Milky Way, and they’re showing you the stars that they’ve identified as one of these ancient dwarf galaxies that have been torn apart. What would that process have been like if you could see a sped-up time from the perspective of either the Milky Way or the dwarf galaxy? What would happen?

Dr. Gay:                      Well, as the dwarf galaxy comes in – and, my favorite of these, simply because it has the most ludicrous name – is the Gaia-Enceladus-Sausage Galaxy.

Fraser:                         Yeah, I don’t approve. I don’t like the name.

Dr. Gay:                      I both hate it and love it.

Fraser:                         I like weird names, but I don’t like it. I don’t know why. I just feel like it doesn’t do it justice – “sausage.”

Dr. Gay:                      It’s true, it’s true, but it’s still delightful. So, as what was probably a once fairly spherical group of stars got too close to the gravitation field of the Milky Way, it would have started to experience what we call tidal forces. This is where the far side of the galaxy and the near side of the galaxy are getting pulled on so differently that it elongates the system. This is the same way our oceans essentially get elongated by the moon’s gravitational pull, except when it’s a galaxy of stars, it just keeps spreading and spreading.

Now, as it got closer, in this case, it appears to have been pulled into a tighter and tighter orbit, and the stars at different distances from the center of the galaxy would not only have experienced different gravitational pulls that elongated the system, but then, over time, the different kinds of orbits would have different orbital speeds, so that further changes how the stars are spread out.

So, we can actually essentially wind backwards the process to see how the stars came in, how, depending on where they were in that initial system, they ended up in different kinds of orbits, and it’s just kind of amazing. It’s like being able to unstir ink in a substance, and there are substances you can do that with, and there are computer models you can, well, unstir the galaxy with.

Fraser:                         Right, yeah. How many of those mergers do we think have happened?

Dr. Gay:                      So, one of my favorite sports – and, I’ve brought this up probably every year for the past several years – is every winter American Astronomical Society meeting since the 2000s, they have come out with another one to many dwarf galaxies that have been discovered as streams of stars. So, we are getting into the tens, it looks like, and while it doesn’t seem like a lot, when you realize researchers are teasing apart individual stars, composition by composition, this is like looking at the population in the United States and rewinding immigration using genetics. They are essentially working out the genetic history of our galaxy through chemistry.

Fraser:                         Or like languages. That’s one that’s really cool. You take a modern language, and then you can work back through the differences and the similarities to other languages – the family tree of languages – all the way back to various mother tongues spoken in various regions. It’s painstaking work, but at least it’s possible now, thanks to Gaia. Did I mention Gaia? Yeah. Thanks, Gaia. How long ago did the last one of these mergers likely happen?

Dr. Gay:                      They’re still happening, and that’s the thing we have to remember, is at any given billion years, a new galaxy could wander close enough to be consumed, and one of the ongoing is it/isn’t it conversations happening in the literature is are the Small and Large Magellanic Clouds on escape velocities, or are they eventually going to get consumed into our galaxy? So, these two little blobs of irregular starry and gassy goodness could, if they’re going slow enough – and, we’re still trying to figure these things out – could eventually become just a new stream of stars thickening the disc of the Milky Way.

Fraser:                         But, the big event is coming in a few billion years, when we collide with Andromeda.

Dr. Gay:                      And, in that case, it’s gonna be a whole lot more than thickening the thick disk. It’s going to be one of these merger events where Andromeda is bigger than we are, but only by about 30%, according to some estimates, and that kind of a merger leaves none of the structures we see today existing the way they exist today. We are going to have the two systems collide, pass through each other, and then, like a damped oscillator, eventually settle into a new configuration that will likely be a more elliptical-shaped galaxy.

In all likelihood, there will be an active galactic nuclei phase where one or both of the supermassive black holes feed on gas and dust that is sent into the core, there’s going to be masses of star formation, and these kinds of dramatic events we can actually study frame by frame by looking all across the sky at a bunch of different merging galaxies.

This is something that Halton Arp did when he put together his catalog of irregular galaxies, which was a series of glass-plate images of systems that looked more and more like dead bugs on your windshield than like spiral galaxies, and these were all the results of systems in different ways coming together with their discs at different angles, with the rotations different to each other, with their masses at different ratios, and we have gotten really good in the past two decades of advancing software technology to recreate all these different shapes and software.

Fraser:                         Huh. It’s funny – so, he built this catalog of weird, irregular galaxies that he didn’t have any explanation for, and now, as you look at them all, they’re all the result of collisions, and each one can be explained in different ways..

Dr. Gay:                      Yes.

Fraser:                         Absolutely fascinating. I’m sure he would have loved to have heard what was uncovered about his work. So, what impact have these mergers with other dwarf galaxies had on the Milky Way? How has our galaxy been changed thanks to all of this additional accumulated material?

Dr. Gay:                      One of my favorite ways – and, everyone’s gonna answer this question with their own favorite things, I suspect –

Fraser:                         Yeah, I have mine.

Dr. Gay:                      – we have what used to be called the Oosterhoff Type 1 and Type 2 globular clusters, and these are sets of globular clusters that are orbiting in different directions and have different compositions, and the stars in both types of clusters are ancient, and now, we are understanding these to be the results of globular clusters getting stolen from incoming smaller galaxies, everything merging together to form our Milky Way, but some of those suckers had a different composition and just came in backwards.

Fraser:                         And so, the orbits of the globular clusters match the direction, the velocity, that the original galaxy came in on?

Dr. Gay:                      Yes.

Fraser:                         And so, some are orbiting in one direction and others are orbiting in the completely opposite direction. It’s funny – I actually did an interview earlier this morning with an expert in galactic morphology, evolution, and stellar motion, and I asked her, “Where do globular clusters come from?”, and she’s like, “We still don’t know.”

Dr. Gay:                      We stole some.

Fraser:                         But, they don’t know –

Dr. Gay:                      How did they form?

Fraser:                         – how they form. Yeah, why are globular clusters? And, she’s like, “We still don’t know. They are ancient. They are as old as the universe itself.”

Dr. Gay:                      But we occasionally see them forming in other systems, and this is what screws us up.

Fraser:                         Yeah. So, mine – you mentioned the thing that you like the best. The thing that I like the best is that the Milky Way is warped, that we have a warp in the galaxy, and this is something that was teased out, I think, of Gaia data by some Chinese astronomers in the last couple of years. If you could see the Milky Way from the side, it would have a pretty significant distortion, and that’s because these mergers aren’t perfect. They’re messy, and some fairly large merger that happened in the past would have thrown a kick into the actual galaxy’s disc shape and cause this warping, which is incredibly tricky to figure out when you consider the fact that we are inside the Milky Way.

The analogy I always use is figure out what your house looks like from inside your house, and the only way you can do that is look at reflected cars as they drive by to give you hints and glimpses of what your house looks like, and it’s not easy. The glass is distorting the shape, and yet, you’re trying to figure out the color, the shape, the number of windows, and the door patterns –

Dr. Gay:                      And there are walls blocking your view in different places.

Fraser:                         Exactly, yeah, and so, it’s tricky. What else do you think are some of the implications for the Milky Way because of the mergers?

Dr. Gay:                      Well, what deeply amuses me is we have been working so hard to come up with a unified model for “If you see this, then you have this much dark matter that is related to the distribution of motions in the spheroid of the galaxy,” and then, a few years ago, we suddenly discovered there’s a few galaxies out there that don’t appear to have dark matter.

And so, that was one of these “Wait, how does that happen?”, and recent models have shown if you have interacting systems that interact just right, as they pass through each other, one of them will end up essentially dark matter-less as the dark matter gets stripped out of the system, and so, while we, in bulk, can say most of the time, when you have these kinds of things coming together, you end up with a galaxy with this ratio of light to dark, but as we look out at all of these dwarf galaxies, we see that some of the dwarf galaxies have massive amounts of dark matter.

Some of them have very little, and all of this comes down to the interaction history of our galaxy, and we are either stealing dark matter or stealing luminous matter, and that is an interesting twist that we hadn’t imagined a few years ago.

Fraser:                         Again, the ability to map out the dark matter locations, the quantities, the densities of it around various galaxies – that science has progressed significantly now so that we have very detailed maps of where the dark matter is surrounding various galaxies, and you just can’t wish the stuff away now in a way that I think people could in the past and just go, “Well, you just don’t understand gravity.” Well, why does the gravity of this galaxy behave differently than the gravity of that galaxy when they’re roughly the same size?

And also, you can see the indexings, you can see how the light is being distorted as it’s passing. Yeah, there are dwarf galaxies with very little dark matter, and then there are blobs of dark matter with very little galaxy in them, and everything in between. This was a big surprise. This is just the last couple of years they’ve really started to make these observations.

Dr. Gay:                      There had been a few isolated papers going all the way back to the early 2000s of looking at this particular dwarf galaxy, it looks like it has a massive dark-matter-to-luminous-matter ratio, but it was just isolated papers, and it’s only through one isolated paper at a time that we’re able to build up this “Yeah, looking at the entire thing, it’s a diverse family out there.”

Fraser:                         Yeah, really interesting. All right, well, thank you, Pamela.

Dr. Gay:                      Thank you, Fraser, and thank you to all of our Patreons out there. Without you, we would not be able to do what we do, we would not be able to pay Nancy, Rich, Ally, Beth, all of the wonderful humans that make this possible, and this week, I would like to thank Thomas Sepstrup, Mountain Goat, Stephen Veit, Burry Gowen, Jordan Young, Jeanette Wink, Kevin Lyle, Andrew Poelstra, Venkatesh Chary, Brian Cagle, David Truog, TheGiantNothing, Will Hamilton, Aurora Lipper, David, Gerhard Schwarzer, Ronald McCoy, J.F.  Rajotte, cacoseraph, William E Kraus, Robert Palsma, Laura Kittleson, Joshua Pierson, Les Howard, Jack Mudge, Joe Hollstein, Gordon Dewis, Frank Tippin, Neuterdude, Adam Annis-Brown, Helge Bjørkhaug, Richard Drumm, Alexis, WandererM101, William Baker, Zero Chill, William Andrews, Andy Cowley, Jeff Collins. So, if you would like to have me mispronounce your name, as Fraser described it last week, please consider supporting our show over at Patreon.com/AstronomyCast. You make the science flow.

Fraser:                         Thanks, Pamela. All right, we’ll see you next week.

Dr. Gay:                      See you next week. Bye-bye.

Voiceover:                  AstronomyCast is a joint product of Universe Today and the Planetary Science Institute. AstronomyCast is released under a Creative Commons attribution license, so love it, share it, and remix it, but please credit it to our hosts, Fraser Cain and Dr. Pamela Gay. You can get more information on today’s show topic on our website, AstronomyCast.com.

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