Ep. 407: Galactic Cannibalism

Remember when we told you that the Universe is a big place, and anything that can go wrong, inevitably does? Today we talk about what happens when galaxies come together. This is particularly pertinent because our Milky Way will collide with Andromeda in the future!

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Female Speaker: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program.  Visit for more information.
Fraser Cain: Astronomy Cast episode 407: Galactic Cannibalism.  Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos where we help you understand not only what we know, but how we know what we know.     My name is Fraser Cain.  I’m the publisher of Universe today.  And with me is Dr. Pamela Gay, a professor at Southern Illinois University, Edwardsville, and the director of Cosmo Quest.  Hey Pamela, how are you doing?
Dr. Pamela Gay: I’m doing well. How are you doing, Fraser?
Fraser Cain: Good. And where are you doing?
Dr. Pamela Gay: I am in The Woodlands, which is a posh suburb on the north side of Houston, where they hold the Lunar and Planetary Sciences Conference every year. So I’m down here to try and absorb all I can about planetary science even though I’m really a stars and galaxies kind of girl.
Fraser Cain: It feels like we just had you there yesterday.
Dr. Pamela Gay: I was at AGU, which is a similar group of people, in December, and yeah. This is a once a year meeting and this year it’s all Ceres and all Pluto. I’m going to sneak in some Mars water, I think.
Fraser Cain: That sounds great. And so maybe next week you can give us a quick update on some of the really cool, interesting discoveries that were announced there.
Dr. Pamela Gay: That sounds like a plan.
Fraser Cain: Okay, right at the source.
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Fraser Cain: So last week we talked about stars feasting on other stars. This week we’ve scaled the whole process up a notch with galaxies feasting on other galaxies. Come for the carnage, stay for the science. All right Pamela. So, now, is cannibalism really the right term? Because someone mentioned in the comments before we started the show that really it’s – They’re kind of borrowing. They’re merging. But there’s absolutely carnage going on.
So what is the situation that leads us to this kind of galactic destruction?
Dr. Pamela Gay: So we actually do scientifically use the phrase, “Galactic cannibalism.” This area of astronomy actually has a lot of really violent terms. There’s galaxy harassment, there’s ram pressure stripping.
Fraser Cain: Bombardment. Walk it off. Yeah.
Dr. Pamela Gay: But galactic cannibalism is the process of one galaxy just systematically chowing down on another one.
Fraser Cain: And so what is the situation that’s going to make this happen?
Dr. Pamela Gay: So normally it’s something like, and we’re doing this all the time with our Milky Way galaxy, there’s a smaller galaxy flying along and it happens to have a trajectory that intersects with our own galaxy or the galaxy that’s going to eat it. And gravitationally it gets pulled in, torn apart, and its constituent matter, its stars, its gas, all that sort of stuff, ends up getting consumed and made part of that parent galaxy; that massive, eating, cannibalistic galaxy.
Fraser Cain: And so I guess when I always imagine this: from the formation of the universe all of the original galaxies formed, and then they’ve been, in various parts, gravitationally attracted to each other, and they sort of do these dances as they come through. And, then they merge over time as they sort of interact again and again.
So did this happen to the Milky Way in the past?
Dr. Pamela Gay: Yeah. So it’s an ongoing process. And, when we look at galaxies, we’re looking at systems that formed in one of two different ways. A lot of the most massive, elderly, elliptical galaxies formed early on in our universe. They formed with the massive collapse of giant halos of dark matter and visible matter. Then you have everything else, which formed more of all these little things that formed and then attracted to one another and formed something bigger, and formed something bigger, and formed something bigger.
So, nowadays, when we look out around our universe, we have these giant elliptical galaxies which are commonly eating just about anything else. Then, we have smaller galaxies that are still large, like our own Milky Way and the Andromeda galaxy that is nearby, and we’re eating dwarf galaxies; little galaxies that are tens of thousands to a few million stars, but are still much much tinier.
And, when we eat these things, it is a fairly hidden process, until you start looking at the ingredient list on the stars that have been eaten. So we can actually look around our Milky Way galaxy and find systems of stars that, in some cases, are orbiting in the wrong direction and are made up of slightly different material than everything else. And you’re like, “Oh, that stuff was eaten from another galaxy.”
Fraser Cain: We see the tails, or the trails, of destroyed galaxies; this sort of smear in the night sky. We just go, like, “That star, that star, that star,” they’re all related, and so they must have come from one gigantic destruction event. But the galaxy itself, the dwarf galaxy, is long gone. It’s now part of us.
Dr. Pamela Gay: And what’s cool is since the Sloan Digital Sky Survey launched several years ago, there has been, year after year after year, different tidal streams around our galaxy identified. And these tidal streams are often dwarf galaxies that, in the process of getting gravitationally shredded by the Milky Way, left behind a smudge of stars that are all made up of the same stuff and are basically that smear of food that the galaxy is still sucking in.
Fraser Cain: And, those are just the ones that have been recently consumed. We actually see a cloud of these smaller dwarf galaxies that were in the process either of whipping around us or about to consume, right?
Dr. Pamela Gay: And, it’s not just these smears of stars. It’s not just the dwarfs. We can actually see remnants of slightly bigger things that were eaten in our past. If we go outside and we very carefully look at all of the globular clusters, we see that some of them are orbiting in one direction around our galaxy, and some of them are orbiting in the opposite direction of our galaxy. And, the compositions of the ones that are going in one way are one composition, and the compositions of those that are going the other direction are a different composition.
And, it appears that a bunch of our Milky Way’s globular clusters actually originated with a different moderately sized galaxy that we chowed down on.
Fraser Cain: Now, those are the sort of smaller ones, but there are some much larger collisions in our future.
Dr. Pamela Gay: Yes. And, we see examples of this all around our universe. The Mice is one of the more picturesque examples if you look at it in the Hubble Catalogue. There’s an entire catalogue of ARP peculiar galaxies, many of which are galaxies in the process of merging together. And when you start to get galaxies that have a ratio of about one to three, where one of the objects is three times bigger than the other object or closer to the same size, so that you have a one-to-one ratio.
So between one-to-one and one-to-three, you end up with these massive, dynamical events during these collisions where things end up looking like tadpoles, big spiraley smudges of twisty arms, and there really aren’t quite enough descriptive words to describe what is going on. But, the result of all of this is you can take a couple of perfectly innocent spiral galaxies and, in the end, have something that is more elliptical in shape, has lost all of this structure, is much more massive, and often, for a while, has a very active galactic nuclei in the center of this mess.
Fraser Cain: And so we look out and we see these galaxies out there, but when we look at the Milky Way, it still really has that fairly nicely preserved disk, even distribution, and doesn’t seem to have that activity. So chances are the Milky Way hasn’t had one of those collisions yet in the past.
Dr. Pamela Gay: No. That’s coming in our future when we collide with the Andromeda and we become what some people call the Milkdromeda galaxy.
Fraser Cain: Yeah. That is the correct name; Milkdromeda. Just so everyone is clear.
Dr. Pamela Gay: But there’s a lot of side effects of these smaller collisions. You end up with things like the disk of a spiral galaxy like our Milky Way will get puffed out. So you end up with a thicker what’s called thick disk of the galaxy. And, that’s just inserting energy into the disk. You’re smearing out all of the different orbits. You end up – In some cases we look out and we see galaxies that actually have warps in the disk. So what should be a nice flat plate instead looks like it’s slightly melted and it has been twisted into something that you wouldn’t want to put food on.
Galaxies get deformed in all sorts of more subtle ways in the smaller collisions, and this kind of makes sense. Smaller things can only have some impact on a system, but they are going to still be bringing in new matter. They’re still going to be bringing in energy. And, all of this going to have an effect. So when our galaxy eats these little tiny dwarfs, we end up with a smear of stars, a few stars that have a completely different chemistry, and a slightly thickened disk to our galaxy.
As you add more and more energy, eating something bigger and bigger and bigger, it deforms your shape more and more. And, as you start to get to that one-to-three to one-to-one ratio, you have complete mutually assured destruction.
Fraser Cain: Right. And, we’re less massive than Andromeda. So we’re the dwarf galaxy falling into Andromeda, although I think we’re closer to that, as you said, one-to-one –
Dr. Pamela Gay: It’s the mutually assured destruction.
Fraser Cain: So cast forward to us on how the next few billion years are going to play out.
Dr. Pamela Gay: Well, things are really going to start to get interesting in four to five billion years. This is going to be when our dark halos start to collide, when we start to see different ram pressure effects. Slowly our systems are going to get these long tails behind them, 5.5 billion years from now. We won’t quite be passing through each other; conservation of angular momentum. It’s going to be more of a whipping around one another.
As we start to merge and collide our two super-massive black holes are going to have to merge or one of them will get flung away. But, most likely, they will eventually merge. And, what’s cool is, in general, stars won’t collide. But, with all of this gas and dust colliding, there’s going to be massive amounts of star formation.
So we’re going to have these long tails of existing stars. We’re going to have massive star formation down in the center of this mess. And, in the end, we’re going to end up with something that has very little gas and dust. Because, it’s either going to get stripped out, or it’s going to get tangled up in star formation. And, so we’re going to have a slowly reddening system with almost no star formation once it’s all over.
So, we’re going to go from two, alive, active, star-forming spiral galaxies to a massive burst of star formation during the collision combining process to a dead elliptical.
Fraser Cain: So, for a while there, we’ll look like the tadpole galaxies, or perhaps some of those ones where you see – And then you’ll see the ones where they’re sort of – Well, I guess the tadpoles are, but they’re sort of away from each other, but there’s tidal tails connecting them together. And then what is an example of what the combined Milky Way and Andromeda will look like? We can see them in our telescopes now, right?
Dr. Pamela Gay: Just a generic elliptical galaxy. We’re just going to be a cigar-shaped blob of stars.
Fraser Cain: Right. Like these giant ellipticals, they almost look like a sphere, like a globular cluster at a galactic scale.
Dr. Pamela Gay: So we do have to be careful, because there are giant elliptical galaxies, which are typically down in the centers of clusters. These are sometimes called CD galaxies, and these are truly massive systems that have grown out of the collision of many many different objects. And then you have your generic, run-of-the-mill, a few times the size of the Milky Way elliptical galaxies that, yeah, it’s bigger than our Milky Way because it took the Milky Way and the Andromeda to form it.
But, they aren’t as giant and they actually have lots of cool structures that are cigar-shaped, are spherical, and everything in between cigar and spherical.
Fraser Cain: But, the thing that I find kind of really interesting is that it’s like, until a galaxy has had one of those catastrophic events, it has that nice spiral structure. But it’s that when two of those come together, then it’s a mess. Because up until that point, it’s like two solar systems colliding with each other, right? And that’s when you get the mess, and you get all of the planets going at strange orbits and things getting smashed into other things and things getting shot out.
So it’s like when you look out and you do see those elliptical galaxies, that’s when you know that they ended with that final catastrophic collision.
Dr. Pamela Gay: And it all comes down to energy. If you think about it, you throw up a thing of pizza dough, which is round, and you spin it and it flattens out. So something like our Milky Way, just because of the nature of how it formed, you end up with this flattened, nice disk.
Now, if you collide two disks at weird angles, you’re no longer going to have a nice, flat disk. So energy and angular momentum dictate the universe.
Fraser Cain: And, I think one other thing that’s really interesting is that by observing these galactic collisions, this was one of the ways that astronomers discovered dark matter, or were able to confirm the presence of dark matter as, most likely, a particle, right?
Dr. Pamela Gay: Right. And so when we look at some of the big collisions that are going on that aren’t just between individual galaxies but are between clusters of galaxies, you have a lot of the same physics. Except, now instead of dealing with particles that are stars and dust grains, you’re now dealing with entire galaxies in the collision, so you have many more stars. The dust is a much different distribution.
And, when we look at the collisions of these systems, we can see when the gas, dust, stars, when that luminous material starts to interact, when it starts to shape out new forms. But, with gravitational lensing, we’re also able to see how the invisible dark matter changes the travel of the light coming behind that cluster, how it lenses that light.
And, we can trace out where the dark matter is, and we see the dark mater is just passing through one another quite happily. Whereas, the luminous matter is colliding quite violently. And it’s in seeing that it’s a mostly collisionless particle that we’ve started to get a better and better handle on what dark matter should be.
Fraser Cain: Right. Yeah. So you’d mentioned briefly about the – There’s a few facts that I find really fascinating. I’d like to talk a bit about the star formation, because we do see some really interesting galaxies out there that have gone through this process of some kind of galactic cannibalism going on there, and that creates these great eras of star formation.
Dr. Pamela Gay: So we see two different situations that lead to star formation getting triggered. First of all, you have two galaxies colliding, harassing one another, basically gravitationally interfering with one another, so that otherwise stable clouds of dust and gas are triggered to collapse. And, what ends up happening is you go from having a nice, stable cloud of dust and gas that’s thermally stable, it’s supporting itself with its heat and gravity nicely balanced, and if you knock that out of balance, gravity takes over. And, once gravity takes over, you end up with massive amounts of star formation.
Now this happens with galaxies colliding, but what’s also cool is this happens when galaxies fall into clusters of galaxies. So you end up with clusters of galaxies on the outskirts will have lots of systems that are undergoing triggered star formation, where as the galaxy falls in it hits the material that has already been stripped out of the galaxies in the cluster. And that, again, triggers star formation.
So all sorts of different things can trigger star formation when interactions happen, but this also means that interactions lead to death. So when that galaxy falls into a cluster, it goes through massive star formation, and then it’s dead. When two galaxies collide, they use up their gas and dust or stream it out behind them, and then they are dead. So, galaxies live longer when left alone.
Fraser Cain: Right. Right. But then they’re a lot quieter and a lot sadder. I mean we only have, what is it, like a couple of stars a year forming here in the Milky Way? While one of these starburst galaxies could have hundreds of stars forming a year. So it’s just a whole – It’s orders of magnitude, and it would be a crazy time to be in one of those galaxies while this is happening.
The other part, I guess, has to do with those super-massive black holes. And you mentioned they might merge, one might get kicked out. What are the physics involved with that?
Dr. Pamela Gay: Well here it’s all going to depend exactly on how the two black holes approach one another. Again, energy is king. And, no matter what the final result is, you’re also going to have a lot of the gas and dust that doesn’t necessarily go straight into star formation. It’s going to no longer have a nice, stable orbit around the center of the original galaxy. It’s going to be trying to figure out how to interact with this new center of mass.
And, a lot of that gas and dust is going to end up spiraling in towards the new center of mass of this new merging system. And, as it spirals in, it forms an accretion disk, and this is where we end up with active galactic nuclei, which is kind of that catch all phrase that we use for super-bright QSOs, quasi stellar objects, quasars, and we also use for the less bright ones that we see in our modern universe.
So basically anytime you end up with a disk of material falling in towards the black holes in the center of these systems, you get a whole lot of interesting physics going on. That disk itself can be quite bright; it can form radio jets coming off of it, massive magnetic fields, again, death and destruction.
Fraser Cain: And, I guess one of the things that’s quite cool right now is we are now post gravitational waves being detected directly by LIGO. And, these are the kind of events that would generate some of the most gravitational waves. It’s these moments where these gigantic black holes are either smashing into each other or whipping around each other. That really bends your space time.
Dr. Pamela Gay: And, we’re still trying to figure out how all of this happens. Because, one of the great mysteries is we look out at all of these merging systems, and we don’t find clear evidence of binary, super-massive black holes quite as we’d expect to. So, there’s still a lot of work to be done. And, a lot of it could be that we just don’t have the resolutions yet to fully say, “This system only has one,” or, “This system has two black holes.”
Fraser Cain: So, let’s kind of go forward into the future for sort of longer periods of time. We see the Milky Way, it’s going to consume a bunch of its dwarf galaxies around it, it’s going to merge with Andromeda and sort of wrap that up maybe 7 to 10 billion years from now.
But, as we sort of continue the clock forward billions, trillions of years, what’s going to happen?
Dr. Pamela Gay: So, we’re slowly falling into a neighboring super cluster and what’s neat, as all of this happens, is it’s a slow gradual merger towards, basically, becoming a giant elliptical galaxy. But, not everything necessarily will end up destroyed in the center. So we can’t know precisely, “Are we going to end up in the center of a super cluster, getting sucked in? Or if a white dwarf is all that is left of our solar system getting sucked into this giant central galaxy? Or are we going to be lucky enough to be on an outer orbit where we get to sit back and watch the destruction of the universe?”
Fraser Cain: Right. Right. But eventually, I guess, everything that is gravitationally connected will either come together or enter some kind of stasis, I guess, if there’s nothing else able to sort of perturb our orbit and move us closer to this galactic cluster. We’ll end up just orbiting it forever, and then everything else will just be zipping away.
Dr. Pamela Gay: Drift away.
Fraser Cain: Drifting away, or even accelerating away thanks to dark energy. So this time, when you think about it, we don’t necessarily live in a special place in the universe, but we definitely live in a special time in the universe, in that there will be a time when this concept of galactic cannibalism goes away.
Dr. Pamela Gay: And, it’s not so much that it goes away as the period between collisions gets longer and longer and longer. So, early in the universe we saw a whole lot more mergers, we saw a whole lot more active galaxies, but as the universe has expanded, there have been greater distances with the voids, and the things that are going to merge have often already merged.
And, so it’s a matter of if you live in a small town, you’re not going to randomly walk into a neighbor as often as you will if you’re in Manhattan. So, over time as our universe expands, as our galaxy falls into our nearby super cluster, as we become part of that Virgo super cluster, we’re going to watch more and more galaxies merge into that CD. And, the rate at which collisions occur will get slower and slower and slower.
Fraser Cain: In fact, I mean, all of the good stuff has already happened. I mean we’re already billions and billions of years after the really exciting times in the universe. Maybe two or three billion years after the big bang was when things were really happening, the stars were forming, galaxies were colliding, it was a mess. Now things are actually pretty settled down.
Dr. Pamela Gay: Yeah. And, eventually we will end up with so little gas and dust around that collisions are going to be plain boring. It’s just mergers of stars into larger systems of stars. So, gas and dust is a limited resource.
Fraser Cain: So, hold onto it while you can. All right, well, thanks Pamela. Have a great time at the conference, and maybe we’ll hear an update from you next week.
Dr. Pamela Gay: That sounds great, Fraser.
Fraser Cain: Thanks for listening to Astronomy Cast, a non-profit resource provided by Astro Sphere New Media Association, Fraser Cain, and Dr. Pamela Gay.  You can find show notes and transcripts for every episode at  You can email us at .  Tweet us @astronomycast.  Like us on Facebook, or circle us on Google Plus. 
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