What are galaxy clusters made of, besides galaxies? How can we use them to understand dark matter? And how can we make pretend ones on a computer? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
Now I'm just a poor country astrophysicist. I grew up in the vast cornfields of the Midwest, and now I live in the vast cornfields of the Midwest. But when when I was 16, my dad took me on a special trip. He took me to New York City. And the country bumpkin that I was, I gawked at everything.
There's just so much stuff. So much stuff and people and things and noises, and you could hardly move your elbows and there were weird smells and there was just activity. It was like being in the middle of a a beehive. It was almost overwhelming. It was just it was just busy.
That was the thing that struck me the most on my first visit to a big city like New York City. I'm pretty sure New York City qualifies as a big city. Is what struck me the most. It was just busy. And years and years later, I did the whole astrophysics and cosmology thing.
I eventually ended up learning about galaxy clusters. Not to be confused with globular clusters or stellar clusters. Those are different episodes, so feel free to ask. I'm talking about galaxy clusters, clusters of galaxies. And when I learned about galaxy clusters, the very first thing I thought of was cities.
So the last episode or a couple episodes ago, was all about voids, which are basically the great Midwestern cornfields of the cosmos. And today, it's about galaxy clusters, the bustling cities of the universe. And like the voids episode, this will be a grab bag exploring all sorts of neat odds and ends and corners and crevices. Thanks to at daviddeponne on Twitter, one eighty five transformer on YouTube, Andre Hsi on email, Filippo s on YouTube, Grace m on email, at Pete Burquette on Twitter, Dean, email, and Martin n on Facebook. Thanks to all of you.
We're all gonna visit the big city, and we're gonna start with with just the definition of a cluster, which is like how you define a city. You gotta make something up to say, yes, that's a city. No, that's not a city. Yes, that one. You gotta make something up.
You gotta make up a definition, so we're gonna make up a cluster definition, a cluster galaxy's definition. Typical clusters of galaxies. Like any say, there's a range of city sizes, and clusters typically run anywhere from eight to 80,000,000 light years across. So just think tens of millions of light years, and you're in the ballpark of a cluster of galaxies, which is large. I mean, no one's gonna shake their head at you for saying that 10 or 20,000,000 light years is small.
That is definitely a big thing. No one's gonna shake their head at you for saying that 20,000,000 light years is big. Either they'll they'll agree, but it's also kinda small when compared to the whole entire universe. Just like cities, if you're in the middle of a city, it seems tremendous. But when you compare a city against, say, the continent or the Earth, they seem kind of puny, and and that's the same kind of visual I want you to have in your head for galaxy clusters.
If you're in the middle of a galaxy cluster, giant. I mean, this is a thing made of galaxies. It is a cluster of galaxies that's gonna be kind of large, and you compare it to the universe. It's very tiny. And just like every city has a slogan, galaxy clusters have a slogan too.
If you see on a billboard, galaxy cluster, the thing that will be underneath galaxy cluster will be the largest gravitationally bound structures in the universe. The largest gravitationally bound structures in the universe. Let's break that down. Structures means structure. It's a thing.
Largest means largest. And then gravitationally bound, this is the key phrase here. This is this is the little asterisk because because galaxy clusters can't claim to be the largest things in the universe because there are superclusters. There's the entire cosmic web, and there's definitely things smaller than a galaxy cluster. But the thing about superclusters, which are clusters of clusters, and the entire cosmic web is that they're not gravitationally bound.
They're not glued together with their own gravity. Superclusters are still in the process of forming, and in fact will never form because eventually dark energy in the expansion universe will rip them apart. And the cosmic web itself is an ephemeral thing. It hasn't been here for very long, and it won't be here for very long. We got lucky to capture it.
Galaxy clusters are here to stay. They're glued together with their own gravity. The same way the solar system is glued together with its own gravity, our galaxy is glued together with its own gravity. Galaxy clusters are glued together with their own gravity, and they are the largest things in the universe to do so. Now compare their size.
10 to 80,000,000 light years across with the size of the universe 90,000,000,000 light years across, and you kinda feel sad for gravity, don't you? The the biggest thing that gravity could make is a tiny tiny dot compared to the whole entire universe. But, hey, let's not rag on gravity. It's got enough troubles on its own. We don't have to make fun of it.
So a typical galaxy cluster, eight tens of millions of light years across, Total mass, total mass of around 10 to the 14 to 10 to the 15 solar masses, which is 1,000,000 times the mass of the sun. And, yes, that seems a little bit excessive. Like, why why are we comparing galaxy clusters to the mass of the sun of all things? Maybe we should use different numbers or different unit system, but this is just what astronomers picked like a century ago, so we're kind of stuck with it. A typical galaxy cluster, a million billion times the mass of the sun.
Each galaxy cluster has around a hundred to a thousand galaxies, which sounds like a lot. Well, I mean, yeah, like, a thousand galaxies is a lot of galaxies, and you add up the weight of a thousand galaxies, you're gonna get a lot of stuff, but that is less than 1% of all the stuff in a cluster. That's right. Less than 1% of the mass of a cluster of galaxies is in the galaxies. Like, these things are defined as groups of galaxies, and yet the galaxies themselves are less than a hundredth of the stuff inside of them.
Again, we picked the name before we figured it all out, so we're stuck. About 10% of the mass of a galaxy cluster is in a hot, thin plasma that threads between the galaxies, and I'll get to that in a little bit later. And so that leaves oh, let me less than 1%, about 10%. That leaves 90% of the galaxy cluster. Has to be something else.
Not galaxies, not gas, has to be dark matter. Galaxy clusters are really dark matter cities. Like, if you see a city from a distance and it's night, you're gonna see what do you see? You see all the lights. You see lights in windows and office buildings.
You see street lights, maybe headlights. And imagine looking at that city from a distance and just adding up all the light bulbs, the mass of the light bulbs. Like, okay. I see see a bright light here, so that's again that's gonna be a certain kind of light bulb. So there's that mass.
That's a headlight. I'll add the weight of the headlight. Oh, a streetlight. I'll add the weight of the streetlight. Adding up all the sources of light is probably gonna be less than 1% of the mass of your city.
Most of your city will be unlit, And that's the case with the cluster of galaxies. Most of a cluster is simply unlit. It is made of matter that doesn't interact with light. And in fact, galaxy clusters are the places of highest concentration of dark matter in the universe. This is where dark matter has been flowing for billions of years, and then all the gas in galaxies have just been along for the ride.
The galaxies themselves inside of a galaxy cluster are buzzing around like crazy. Hundreds of kilometers per second, which is fast. And imagine a galaxy, entire galaxy, you know, home to hundreds of billions of stars moving that fast. Some of them get up to a thousand kilometers per second. They're all buzzing around like bees in a beehive, like people in a sages buzz buzz buzz buzz, go go go go go always on the go.
The gas itself, this this hot thin plasma gas, is between the galaxies. It's denser in the center, of course, and then thins out, thins out, thins out. And the dark matter is just the dark matter. It just kinda sits there. There's no well defined edge to a galaxy cluster.
It's not it's unlike a city. Like, a city has city limits, but then, you know, people live outside those city limits, and maybe they'll have the same postal code. Maybe they go to work in the city. So galaxy clusters, we can define an edge, and yet there's still stuff outside that edge. So when I say a galaxy cluster is, you know, tens of millions of light years across, that's to where the galaxy cluster gets thin enough, the dark matter and the gas gets thin enough that you can basically kinda call it.
Like, yeah, we've gone far enough outside the city. We're starting to see some cornfields here, so maybe we should put a limit here. Now some more about this gas. 10% of the mass of a galaxy cluster is in a gas. It's called the ICM or the intracluster medium.
I'll break that down for you. Intra means inside, cluster means cluster, and medium means stuff. So the inter cluster medium is the inside cluster stuff. What is it? It's hydrogen and helium, you know.
Like, everything else in the universe, no surprises there. But here's the surprise, it's insanely hot. It is so hot. It is tens of millions of Kelvin. Like, that is I mean, it it's just hot.
It's just hot. That's, like, hotter than the surface of the sun. It's hotter than the solar corona. It's hotter than hot stars. This is an incredibly hot gas.
So hot that it emits Bremsstrahlung radiation. Remember that fun word from the radiation episode that I still failed to pronounce? Thank you to all the German speakers who corrected me, and sorry your lesson didn't stick. Bremsstrahlung, I think, is the closest I'm ever gonna get. Bremsstrahlung radiation is is this gas is so hot and it's colliding with itself, it emits X rays.
Like, just imagine a ball of gas so hot and so energetic just on its own, it generates X rays. That's what we're talking about, and there's tons of it, more than tons of it. There's a lot of it, like 10 to the 14 solar masses, a hundred million million times in the mass of the sun, spread out over tens of millions of light years, but it's not dense at all. Yes. It's hot.
Yes. There's a lot of it, but that's a lot of space. And there's a word a phrase here we use in physics to describe density. I mean, we we have density too. Don't get me wrong.
But but there's another interesting phrase that pops up here when talking about the intracluster medium, and this phrase is called the mean free path. The mean free path. It it says the average distance you go before you bump into something. So imagine if you're out in the middle of one of these Midwestern cornfields and you start walking, you're probably gonna have to walk pretty far before you bump into someone. So your mean free path, the average distance you can go and still be free and not bump into anyone is pretty big out in the Midwestern cornfields.
But, say, the streets of Manhattan, you're probably not gonna go very far, like, two steps before you bump into someone. Your mean free path is gonna be much, much shorter in the in the streets of Manhattan than it is out in the Midwestern Cord fields. So, of course, physicists can't get away without can't do anything without inventing a jargon word for it, so here we are, mean free path. A mean free path is important because the intercostal meridian is made of stuff, and this stuff bumps into each other, emits radiation, interacts. And so the mean free path gives us a a kind of sense of how the activity level inside of this intracluster medium.
Get this, the mean free path of the gas inside of the intracluster medium is a light year. A light year. Imagine you're you're a little proton. You're a little helium nucleus, and you're just buzzing around super fast, super hot, and super lonely. You're gonna travel, on average, a light year before you bump into another proton or another helium nuclear or anybody.
A light year at a time between interactions. That is a thin gas. Super hot, but super thin and super lonely. Now this gas has something strange about it that we're not exactly sure how to solve. The gas is obviously denser in the center because it has had more time to sink down, so the core of galaxy clusters has more galaxies, also has more dark matter, and has more gas.
And when this gas is denser, its mean free path is shorter, so there should be a lot more interaction, so it should emit a lot of radiation. And if it emits radiation, it's releasing energy, so it should cool off. And yet, the gas in the center of galaxy clusters is actually pretty dang hot. This is something we call the cooling flow problem. The gas in the center of galaxy cluster should be nice and cold, but something's keeping it warm.
We're not exactly sure what we think it has something to do with active galactic nuclei. These are the giant black holes inside of galaxies, and when they feed, some of the gas gets pulled into the black hole while some whips around and then gets ejected out before falling in. We see these all over the place. These are the quasars, the blazars, the everybody's very, very energetic events. We think, maybe, that these events these feeding events from supermassive black holes are enough that in the center of galaxy clusters, there's gonna be, like, a big galaxy, and the big galaxy typically has a giant black hole inside of it.
And every once in a while it feeds and it ejects energy out, way out radiation matter, the whole deal, blasting out of the host galaxy all the way out into the intracluster medium. We actually see this. We actually have literal images of this, of, like, of jets blasting out of galaxies. They inflate bubbles that are tens of thousands of light years across. The bubbles rise into the intercluster medium and eventually disperse, and it's heating it up.
Now, of course, we don't have the full physical picture because it's kind of complicated. You know, giant clusters, tiny black holes, most powerful engines in the universe, etcetera, etcetera, complicated physics. But we think this is the general picture that which is it's that's actually fun to think about. These giant black holes sitting in the center of galaxies can heat a million million solar masses worth of gas spread throughout millions of light years and keep it warm for billions of years. That is something pretty cool.
Again, not fully understood. We're working on it. Please have patience. Now the the galaxy clusters themselves sit in the cosmic web because everything, all the stuff sits in the cosmic web. In the cosmic web is this filaments of of galaxies and structures that looks like a giant web exactly as the name suggests.
The galaxy clusters are at the intersections of walls and filaments. These are the high density. These are the nodes. These are the knots in the cosmic web. And so they're gonna be the highest density places, the the most activity places, and the the filaments themselves, these long, thin ropes of galaxies, are feeding the galaxy clusters.
So these are like the highways leading into the major cities. The galaxies inside of a filament, inside one of these long thin ropes, the galaxies are literally moving. They're trucking along. They're blasting in. They're falling into the galaxy clusters.
Over time, the galaxy clusters are getting bigger and bigger as galaxies fall into them. We, us in the Milky Way, we're on our way to the Virgo cluster. Now we'll never reach it because accelerated expansion, dark energy, that's a different episode. But if it weren't for that, eventually, we we're on the we're on the road. We are on the way to feeding our nearest galaxy cluster, the Virgo cluster.
And so over time, galaxy clusters grow. They accumulate gas. They accumulate dark matter. They accumulate galaxies, and they get bigger and bigger with time. Eventually, that growth will stop, and the cosmic web will dissolve, and all galaxy clusters will be ripped apart from each other.
And the universe will just consist of a bunch of isolated galaxy clusters and groups, and that's it. And then these galaxy clusters were slowly or rapidly spread apart from each other. But that's way in the future. We don't gotta worry about that now. There's one galaxy cluster in particular that is especially famous.
It's the Coma cluster. It's so called because it sits in the constellation Coma Berenices, which is otherwise one of the most unremarkable constellations out there, except for the fact that if you look in the direction of that constellation you're looking at, a monster of a galaxy cluster. It's 320,000,000 light years away. It has at least a thousand galaxies. It's a big one.
As clusters go, as cities go, it's a big one. And the Coma Cluster is where we got our very first hint on the existence of dark matter. And this was way back in the nineteen thirties. There's astronomer Fritz Zwicky, who is perhaps my most favorite astronomer simply because of his name and his fondness for bolo ties. He was studying the coma cluster.
He was studying the galaxies. And back then, all we had, we didn't have X-ray astronomy. We just had galaxies. We we looked in this direction and we're like, man, there's a lot of galaxies associated with each other. And he was studying the motions of these galaxies.
Now, obviously, you can't do this in real time because even though the galaxies are fast, it's kinda hard to pick out the motion and watch them move. But you can look at their redshift and blueshift. You can see how fast they're going, and you can get some sense of the direction of motion. And he very quickly realized that the galaxies in the coma cluster are moving way too fast. The galaxies in the coma cluster shouldn't be gravitationally bound.
They should have ripped themselves apart eons ago. So this is a problem. Given the speed of these galaxies, either the coma cluster just happened to recently form by accident, and it's just like a random association of galaxies, and it's a very temporary thing. We just happen to get lucky that this giant cluster right next door is a very rare thing. That seems kinda weird.
We don't like coincidences in astronomy. Or something else is keeping the cluster together. Something else is keeping the cluster glued together where the galaxies can have very very high speeds and the cluster doesn't disintegrate it stays glued Good old Fritz didn't really know what to make of this. He's German, so he called it, Dunkel Monterey. Sorry, again, for my pronunciation.
It means dark matter. He he dropped the subject. He he wrote the paper. He's like, I've got no clue what's going on. There must be something else up there in the coma cluster, but I've got other fish to fry.
See you later. And no one really picked it up. Over the decades, people would study other clusters, come to the exact same conclusion. So it's either every cluster in the universe is just totally lucky, and we just happen to see it in this very temporary phase or there's something gluing the clusters together. Then in the nineteen seventies, Vera Rubin did this whole thing on galaxy rotation curves, discovered the bigot piece of evidence for dark matter inside of galaxies, and, uh-huh, the light bulb went off.
If there's dark matter in galaxies, there's probably dark matter in clusters of galaxies. I mean, that's where we first spotted it. We just called it dark matter as a placeholder, and now it was a thing. And the same thing that was in galaxies was in clusters of galaxies. Dark matter does the work of keeping the galaxy cluster together.
That is what makes it gravitationally bound. Otherwise, the galaxies are moving way too fast, and it should have just disassociated billions of years ago. Dark matter keeps clusters together, just like Patreon keeps us together. Keep keep contributing, and please keep keep going to patreon.com/pmsutter. It is your contributions that keep this show going and keep us as one big, happy, science loving family.
This isn't the only way that clusters of galaxies help us understand dark matter. Of course, there is gravitational lensing. Like, you have a lot of stuff. You have a million billion suns worth of stuff in a relatively small volume compared to the universe. And so that's gonna bend a lot of light.
These are gonna be the biggest, strongest lenses, ways of gravitationally bending light in the universe, and we see this all the time. I mean, just look up a picture of a galaxy cluster. You will see a bunch of galaxies. Duh. But you'll also see arcs in squiggles, in weird shapes and colors.
What you're seeing in those arcs and squiggles and in weird colors are galaxies behind the cluster whose light has been distorted as it tried to pass through the cluster. This is gravitational lensing, and clusters of galaxies are the best gravitational lenses there are. So we can use this both to understand dark matter because the amount of dark matter in a galaxy cluster tells us, you know, it affects how much lensing there's gonna be. We can also do, really cool magnification. We can use clusters of galaxies as giant magnifying lenses and see stuff behind them that would normally be too dim to see, but the light gets magnified, and we can see it as pretty cool trick.
If you ever see a news story about, like, furthest galaxy ever discovered, 99 times out of a hundred, they use gravitational lensing from a nearby cluster in order to get that image. And there's one other way that galaxy clusters help teach us about dark matter, and that's through their collisions. That's right. Galaxy clusters evolve. They don't just, like, slowly accumulate galaxies and gas and dark matter.
Sometimes clusters crash into each other, and when they do, they're the most energetic events in the universe. Anyway, If you have a million billion suns on one side and a million billion suns on another side, and they're traveling at, say, I don't know, a thousand kilometers per second, and they crash into each other, that's gonna release a lot of energy. This is pretty rare. About 80% of the galaxy clusters we see are nice and round and spherical, implying that they finished forming a long time ago, anywhere from five to 10,000,000,000 years ago. And now they're just slowly accumulating stuff, but, you know, their violence is in their past.
But about 20% of galaxy clusters are just straight up gross looking. They're all distorted and wonky and elongated and lumpy, like, just just gross. And this means they had a recent merger that recently another cluster slammed into them, and they haven't quite sorted themselves out yet. One of the most famous examples of these collisions is something called the bullet cluster, where a smaller cluster plowed through a bigger cluster. This thing is about 4,000,000,000 light years away.
And when you look at this with three different views, you get a very, very strange picture. If you just look at the galaxies, you know, visible light, then you see a clump of galaxies on one side and a clump of galaxies on another, which is about what you would expect because the galaxies are so tiny compared to the whole entire cluster that they just sail on through. When you look at the gas, the X rays, the bremsstrahlung emitted by the gas, it's all tangled up in the center, and there's shock waves and cold fronts, all sorts of cool stuff. Whenever you have two big blobs of gas slamming into each other, there's all sorts of cool physics. And that's on display in the middle.
So the gas is all tangled up, hasn't sorted itself out. And then you use gravitational lensing to look at where the mass is. Like, where is the rest of this 90% of this stuff? It's not caught up in the middle. It's out there with the galaxies.
But the galaxies obviously don't have enough stuff. You just made a map of the dark matter. And the bullet cluster is very important for us understanding the dark matter because it gives us a clue about what the dark matter is made of or at least how it behaves. Dark matter isn't behaving like a gas. Gas interacts with itself.
You know, hydrogen interacts with hydrogen, talks to hydrogen, gets tangled up with hydrogen, and so you get all this stuff tangled up in the center, but that's not where the dark matter is. The dark matter is out floating at the edges, which means dark matter doesn't talk to dark matter. Dark matter doesn't talk to light, doesn't talk to normal matter, and it doesn't talk to itself. It passes through itself. If I have a lump of dark matter and another lump of dark matter and I slam them together, nothing happens.
They just sail on through like they didn't even know they existed, and that is on display in this bullet cluster. This is telling us that dark matter doesn't interact with itself, or if it does, it does so incredibly weakly. When we first made this observation of the bullet cluster back in 02/2004, this was a big piece of evidence that dark matter is some sort of exotic particle that does not interact with itself. Because all other ways of doing dark matter of, like, adjusting what gravity is made of, or may have it be, like, black holes and and other stuff we just can't see. It just doesn't work.
It can't explain the bullet cluster like anything else can. It just can't explain the bullet cluster like particles that don't interact with themselves or anybody is able to explain. There's one more thing about galaxy clusters that I need to tell you about, and I need to tell you about it because it's a really, really fun phrase to say. It's the Sonyaev Zel'dovich effect. Now this is the bomb you drop at a party.
Like, if if if the room's quiet, conversation is lulling, no one really knows how to proceed, You don't really know where to go from here. You just say, like, hey, folks. Let me tell you about the Sonia of Zaltofitch effect. I guarantee ears will prick up and eyes will turn towards you. I don't know what the facial expressions will be, but at least you'll have their ears and eyes with the sunniest Zelda witch effect.
And it has to do with the hot gas in the clusters. The cosmic microwave background, this leftover light from the very early universe, has been sailing through the universe for billions of years. It's very, very cold. This radiation is very it's in the microwaves, three Kelvin, three degrees above absolute zero. It's just cold cold cold radiation.
And it permeates the universe. It's everywhere. And sometimes this radiation passes through a galaxy cluster because, you know, it's radiation and soaking the universe. Some parts of the universe are inside of galaxy clusters. And it encounters this hot gas.
And every once in a while, one of these super lonely but super fast protons or helium nuclei that make up the intracluster medium will slam into a cosmic microwave background photon and energize it, boost it, make it hotter. And this effect is called the Sonya Zel'dovich effect after the two Russian scientists who figured it out. Rashid, a Tsarnaev and Yakov Zel'dovich, if you're curious about their first names, which you should be because they're awesome people. We can see galaxy clusters when we make maps of the cosmic microwave background because they show up as tiny little hot spots. Now it's not every hot spot.
It's it's a very, very subtle effect, a very, very tiny effect, but we're good at measuring the CMB by now. So we can do it. And we see it. We're able to map thousands of galaxy clusters that we wouldn't be able to pick out with surveys or with X-ray instruments purely through this sun yave, zeldovich effect. Now, of course, in astrophysics circles, everyone's super cool about it and says s z effect.
So you might wanna switch to that if you're in a room full of physicists instead. But if you're not in a room full of physicists, you gotta use the full phrase, Zeledovich effect. And you can thank me for that later. Thank you so much for listening. I would like to thank, speaking of thanks, my top Patreon contributors this month, Robert r John, Matthew k, Helga b, Justin z, Matt w, Justin g, Kevin o, Tungen m, Corey d, Kirk b, Baragay, Nerdwriter, Chrissy, and Eric m.
It is your contributions plus everybody else's. Something like 250 people contributing to this show. I can't believe it. It is so I'm so grateful. That's patreon.com/pmstar if you wanna help keep this train rolling too.
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