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Our galaxy series continues, on to spiral galaxies. In fact, you’re living in one right now, but telescopes show us the various shapes and sizes these galaxies come in. Thanks to JWST, we’re learning how these spirals got big, early on in the Universe.
Transcript
Human transcription provided by GMR Transcription
Fraser Cain:
Astronomy Cast, Episode 719 – The Galaxy Series: Spiral Galaxies. Welcome to Astronomy Cast, 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. I’m the publisher of Universe Today. With me, as always, is Dr. Pamela Gay, a senior scientist for Planetary Science Institute and the director of Cosmo Quest. Hey Pamela, how you doing?
Dr. Pamela Gay:
I’m doing well. When folks are hearing this episode, you’re going to be off in Japan, and that is amazing.
Fraser Cain:
Yeah. Yeah, we’re making up for that trip, so four years ago in 2020, I booked a trip – actually in 2019 I booked a trip to Japan with my son, and then COVID hit. And things got pretty dicey, and so we decided to cancel the trip. And it’s been four years, and now what’s COVID? Who remembers that anymore? So, we’re gonna do it again. And now he’s older, and a lot wiser. And I think it’s going to be a really fun time, so yeah.
By the time you listen to this, we’ll have been in Japan for a week. And people are like, “Are you going to do this? Are you going to do that?” I’m like, “I have no plans.” This is purely vacation. If I runaway across JAXA as I turn a corner in some neighborhood, then sure, I’ll walk in. But I’m not booking things, doing interviews, any of that. I am gonna bring a camera, but apart from that, no. This is just purely fun. I wanna eat some tasty sushi and noodles. I want to walk around cool parks and temples. I wanna ride bullet trains, and I want to meet people there. So, yeah.
Dr. Pamela Gay:
I have to say, I haven’t found sushi there that was any better than what I’ve gotten in the U.S., other than you can get fugu there, which you can’t get in the U.S.
Fraser Cain:
No thanks.
Dr. Pamela Gay:
But the ramen. I had ramen that is the stuff of dreams, and may you find such hole –
[Crosstalk]
Fraser Cain:
I’m looking forward to that.
Dr. Pamela Gay:
—in the wall ramen places.
Fraser Cain:
Yeah. Yeah. That sounds great. Our Galaxy Series continues. On to spirals. In fact, you’re living in one right now, but telescopes show us the various shapes and sizes these galaxies come in thanks to JWST, we are learning how these spirals got big early on in the universe. All right Pamela, spiral galaxies. And this is obviously familiar territory because we live in one.
Dr. Pamela Gay:
We do, although living in it has made it particularly challenging to study. Like we figured out, there is a bar in the center of our galaxy only within the past couple of decades. We still see every few years, they change their mind on exactly how far we are from the core of the galaxy.
Fraser Cain:
How many arms the Milky Way has is – the last episode, we talked about different controversies that are still unfolding, and one of those is how many spiral arms does the Milky Way have. This is a question that – is it two? Is it four? Is it two but then two other kind of arms that are broken off of it? It is a tricky question because you’re embedded inside. Like quick, imagine yourself in a random house. What color is the paint on the outside of your house?
Dr. Pamela Gay:
It’s a challenge.
Fraser Cain:
It is a challenge. Yeah. Yeah, maybe you’ll see it in reflected windows of cars as they drive by.
Dr. Pamela Gay:
And what makes the challenge all the more fun is we have this Andromeda Galaxy looming so very close, and at first glance, it seems to be so much bigger than we are. And yet every time we revise our math estimates, it seems that our galaxy and Andromeda get closer and closer in size. And so, it just turns out trying to understand things that take more than one field of view of a telescope to look at is really hard. Really hard.
Fraser Cain:
Yeah. I love those images of what Andromeda would look like if it was bright like the full moon, and you could see it. It is the size of – what is it? Like nine full moons in the sky. Gigantic.
Dr. Pamela Gay:
Yeah. Yeah. And then there’s also the challenge of the size of galaxies varies depending on what color of light you’re looking in. And this has been one of the challenges with classifying galaxies, especially spiral galaxies. So, the old school, still taught, tried and true way of classifying spiral galaxies is the Hubble Tuning Fork diagram, and in this diagram, you have ellipticals of varying roundness to flatness that eventually become what is called a lenticular galaxy, where you have a nucleus in the center and just like a disc of no structure around it.
And then, it forks, thus the tuning fork part of the analogy. And the arms on the galaxies for the spirals and the barred spirals just get more and more unwound, but it turns out, if you look at galaxies in different colors of light, you’re able to see their arms in different degrees. And so, when it comes to trying to figure out how do we write software to classify galaxies, how do we get human beings to classify galaxies, it is a glorious disaster.
Fraser Cain:
Right. Right. And I mean it’s just a mess because galaxies are weird. Yeah, it’s not humans. It’s the galaxies’ problem.
Dr. Pamela Gay:
Go galaxies.
Fraser Cain:
Right. Yeah. Keep being weird, galaxies. So, then how do they form? I mean we look at these – I mean there’s some beautiful examples – the Whirlpool Galaxy, the Pinwheel Galaxy. They’re nice face-on, spiral galaxies where we look just right down onto them, as well as ones that are farther away that are less famous that are equally as beautiful and distinct. How do we get this weird shape in some of the structures that we see?
Dr. Pamela Gay:
So how exactly you go from either blob of mass to spiral galaxy or a whole bunch of dwarf galaxies merging together, which is how these probably form, but the universe likes to have exceptions to every rule, and I’m just going to throw that out there. There are always exceptions. It looks like we get spiral galaxies through the merger with the correct angular momentum coming together to set things in a nice coherent spiral.
Then it gets trickster-y though, because spirals come in different varieties separate from just how much of their arms splayed out. We have what are called flocculent spiral galaxies, which are spiral galaxies when you look at them, you see there is what appears to be feathers – flock, flocculent – of spirally bits all throughout them, but there isn’t a clearly defined pair or multitude of arms. It’s just like arm bits all the way down. Then you have galaxies usually that have a companion. We think that it’s the gravitational interactions that drive the spiral density waves that create these grand design spirals, which have two arms. Only two arms, perfectly formed –
[Crosstalk]
Fraser Cain:
And the number shall be two. Two.
Dr. Pamela Gay:
— [inaudible – crosstalk] [00:08:44]. And the number shall be two and only two.
Fraser Cain:
Two was the arms. Yes.
Dr. Pamela Gay:
And so, going between these extremes is every possible version of messy and glorious. And then we see some spiral galaxies have rings in their centers, have bars in their centers. And, again, this is all driven, we think, from the gravity of companions. And then, we have things like the Seyfert 1 and 2 Galaxies that have active galactic nuclei that are shooting out jets of radio waves. Spirals are just out there trying to show off they are the drama queens, they are the pageant queens.
Fraser Cain:
Right. You know, the peacocks of the galaxy world. So, this shape – I mean it really looks like someone is winding up a bunch of stars from the middle and you get these spiral arms that form. What are the spiral arms?
Dr. Pamela Gay: They are actually just places where material lingers as it goes on its orbit around and around a galaxy. It’s not that galaxies have solid disc rotation where those arms are intact and the whole thing is bulk rotating like a pinwheel. That is not happening. I just want to make that very clear. The structure might look like a pinwheel. It is not rotating like a pinwheel. So, what’s happening is as material goes around and around the core, there are regions that have higher density. The regions that have higher density accelerate material towards them, so it gets there faster and then holds onto it, so it slows down as it exits.
So, the amount of time that material stays in the arms is increased compared to the amount of time that it spends on the other parts of its orbit. So, you can have material zoom into – I don’t know why I said that like that. You can have material that zooms into a galaxy’s arm, passes through ever so slowly, interacting a lot, star formation gets triggered in arms, and then passes out the other side until it gets to the next arm, rinse, and repeat.
Fraser Cain:
Yeah. So, analogy that I love to use, like imagine you’re in a balloon above the Super Bowl. And you’re looking down, and the wave is happening. I don’t know if they do the wave at the Super Bowl, but imagine the wave is happening. And so, you’ve got human beings standing up, shaking their banners, cheering, and then sitting back down again. And you’re seeing this wave propagate through the entire arena, and from your blimp view, it looks like something is turning inside the stadium, but it is not what’s turning. It is the people standing up and sitting back down creating this illusion.
And that’s the same thing that’s happening with those spiral galaxies. Now the whole galaxy can be spinning. That’s a separate thing. But those spiral arms that you’re seeing are these density waves that are just rotating through as all the stars are doing the wave. And they take their time. They have their turn in the arms and then times when they are not in the arms.
Dr. Pamela Gay:
And to be clear, the majority of material in a spiral galaxy – not all, there’s always exceptions. That’s just going to keep coming up. The majority of material in a spiral galaxy will be orbiting all in one direction, like cars on a racetrack, should in theory, all be going in the same direction. And what we’re seeing is all these things that are going more or less in the same direction are just lingering longer where there’s a higher amount of mass to pull them in and hold onto them as they try to continue their orbit.
Fraser Cain:
But we clearly see these star forming regions in the spiral arms of these galaxies, so what’s that about?
Dr. Pamela Gay:
So, if you think about it, star formation gets triggered through interactions, through shocks, through something taking a nice stable cloud of gas that is supported through the balance of gravity inwards and thermal pressure outwards. Doesn’t take a lot to knock that kind of a cloud out of equilibrium.
So, as these nice friendly clouds enter the region of crowding, the probability that something that got there before them is going to have a supernova go off, the probability that a couple of these clouds are gonna interact with each other and knock each other out of equilibrium is a whole lot higher than when that cloud is all by itself in the space between arms. So, when these clouds get to the high-density region of the arm, they tend to get knocked around and that knocks them out of that very careful, thermo-gas dynamics versus gravity balancing act. And you get star formation.
Fraser Cain:
Right. So, parts of the cloud are pulled in and then you get the densities that can begin and trigger the star formation.
Dr. Pamela Gay:
Or it could literally just be the shock wave from the supernova hitting the end of these clouds, and there’s a lot more supernova going off in the arms where you have a lot more star formation. And supernovae go off when you have star formation, and those first giant stars die.
Fraser Cain:
Right. Right. It’s this cycle that just gets rolling. It’s crazy. Like I just sort of imagine this wave sweeping past, and as the wave is sweeping past through space, you’re getting clouds of stars start to form in this. And then supernova are going off, and this triggers more star formation. And then the wave passes, and the fuel is depleted, and you have less stars in that region, but now the next regions gets filled with stars. It’s a very – I don’t know, very evocative concept to think about.
Dr. Pamela Gay:
A better analogy might be a traffic jam. I don’t know if you’ve ever been driving a long on a road trip, full tilt boogie, and all of the sudden, three miles ahead there is an accident on the complete other side of the highway. There’s no reason for your side of the highway to slow down.
[Crosstalk]
Fraser Cain:
Yeah, and yet people do.
Dr. Pamela Gay:
But as it turns out, because human beings are human beings, they will race forward and end up piling up and then when they get close to that accident, they’re like, “Must look. Must look.”
Fraser Cain:
Must. Yeah. What is it? How many police cars?
Dr. Pamela Gay:
Right. And so, you end up with this compression wave triggered by looky-loos. And that causes a compression of cars in that one place. Well, here it’s the gravitational wave of looking at the car accident that’s causing the compression and the lingering. In the galaxy, it’s the gravitational pull of all the cool stuff going on that has mass and is holding you in place.
Fraser Cain:
So, once again, JWST has joined in the hunt for galaxies, and because these things are fairly large and fairly bright, it’s seeing spiral galaxies early on in the universe. So, give me some surprising discoveries about spiral galaxies thanks to JWST?
Dr. Pamela Gay:
So, we thought that they would come very slowly into being, it would take a couple billion years for them to exist built up through the slow aggregation of smaller systems into larger systems. And it turns out, something happened. We don’t know exactly what happened. JWST is still looking. And hundreds of millions of years, not a billion years, hundreds of millions of years, we’re already starting to see spiral structure. It’s not perfect, at least not what we can see through JWST, which admittedly isn’t that many pixels across. But still, it’s enough that we can see the spiral structure.
And so, it turns out that somehow these things are forming faster and earlier than we thought through means that are still being defined. And there’s so much to figure out. Like if you look at the velocity curve of a dwarf, irregular galaxy, they have the same velocity curve structure as a spiral galaxy. So, these dwarf irregulars that look like dead bugs on the sky, have stars that are mostly going around and around in one direction in a known way related to dark matter.
And then we see spiral galaxies. And so, how are these things merging to get us bigger systems? Are they forming just big and spiral? We don’t know. We’re figuring all this out. It’s a really cool time to realize everything we knew was wrong, and we get to start over and try again.
Fraser Cain:
Right. Right. And the other thing that – this is fairly recent news. I don’t know if you have been following this story, but they’ve found that the galaxies have bars as well early on.
Dr. Pamela Gay:
Yes. And that implies companions.
Fraser Cain:
Right. Right, which was what you were talking about earlier. That there’s some kind of interaction between the galaxy and its companions leading to this bar forming in the middle. What is this bar?
Dr. Pamela Gay:
There are so many different papers that don’t say the same thing. So, what it is, for reasons that have many explanations and I’m not going to make a personal opinion right now because someone will send me a nasty letter. There are galaxies, including our own, that have a companion and have a structure in the center that is linear and radiating out from the black hole, and then these spiral structures appear to spiral off of the ends of this bar. That companion is the consistent part. Exactly how the barred structure forms, there’s lots of theories. I’m just going to leave it there and not deal with the letters in my inbox.
Fraser Cain:
Yeah, so it’s a couple of things. 1). It is that about two-thirds of galaxies have bars, and they appear to come and go over time.
Dr. Pamela Gay:
Yes. It’s a transient phase, which is consistent with the companion galaxies coming and going, changing in distance, getting consumed actively.
Fraser Cain:
Yeah. Yeah. And so, you can get some event that causes the bar to buckle, to collapse in on itself and disappear again. And then other times, the bar will start to spread out and stretch out, and the arms end up at the end of the bar. So, it’s a weird thing. Spirals have them, and yet has – I mentioned this that now there’s observation that they’re seeing these spiral bars in galaxies that are under a billion years old, like you should not have seen these mature structures in galaxies and yet, there they are. So, once again, the universe is speed running its large-scale structures, its more mature structures, and this is a surprise.
Dr. Pamela Gay:
And what I’m really loving is we already knew quasars’ active galaxies were much more common in the early universe. We haven’t been able to really make out consistently the structure around them. Studies that use Sloan Digital Sky Survey to do extremely statistically rigorous looks found that there were the same fraction of mergers among quasars as non- quasars, so there’s just something special in the systems with quasars that causes them.
But there’s something of the early universe. There was more gas then. There was more material then. And so, we have all of these weird things that were high energy events creating amazing forces. There was more stuff around to do the mergers that hadn’t formed large galaxies yet. It was basically the pottery waiting to be formed.
Fraser Cain:
Right. We talked a lot about dark matter in the last episode, and I think we should definitely talk about dark matter as it relates to a spiral galaxy as well. So, what role does dark matter play in the behavior of the galaxy?
Dr. Pamela Gay:
It changes how they rotate, or it changes – I guess a better way to put it, how the stars at a variety of different distances orbit around the galaxy. This was one of the things discovered by Vera Rubin, and what’s remarkable here is Vera Rubin was trying very hard to do nonconfrontational research. She moved away from other topics because she was like, “Nope. Don’t want to deal with the politics. Just wanna do science.” And she quite accidentally discovered that as you move out from the core of a galaxy and you get more and more material inside your orbit, it was expected that things would be going at lower and lower orbital velocities.
Fraser Cain:
Like the solar system.
Dr. Pamela Gay:
Like the solar system. And instead, what happens is it just flattens off. Just flattens off. So, the outer parts of galaxies, out the greatest distances we can see, beyond a certain point, everything just keeps rotating at the same rate. And this is because the distribution of dark matter is such that it’s counter balancing what we see with the baryonic luminous matter and changing the rotation curves. And so, we’re essentially trying to map out the distribution of material we can’t otherwise see by looking at the rates at which stars, globular clusters, clouds of neutral gas are going round and around our Milky Way.
And poor Vera Rubin who was trying to do nonconfrontational research discovered this, ended up having to spend about a decade proving that she was right. Along the way, she demonstrated that work done in the ‘30s by Fritz Zwicky on galaxy clusters was the exact same effect, and then the poor women never got the Nobel Prize for everything she went through. She got many awards, but it is generally seen as a great oversight that she didn’t get the Nobel Prize for what she did.
Fraser Cain:
I wanna sort of just reiterate this discovery because I think it is so foundational, and you can’t hear this and roll your eyes at dark matter. Which is what I see a lot of in the comments, so if you’re like, “Astronomers just make up this thing called dark matter for blah, blah, blah.” You know what? No. No. No. Absolutely not. That is incorrect, and let me sort of give you this insight, right? You measure – like here in the solar system the Earth is going at 30 kilometers per second around the Sun. Neptune is going five kilometers per second around the Sun. There is this drop off in the velocities of the planets as you get farther from the Sun. It is this steady line going downward that measures the velocities.
You look at a galaxy – yeah, close to the center of the galaxy, the rotation rate is increasing, and then you hit this point where you then, as you measure outward, it’s like – what is it? Two hundred fifty kilometers per second, and a little farther away it’s 250 kilometers per second. Little farther away, you know –
[Crosstalk]
Dr. Pamela Gay:
Still.
Fraser Cain:
—250. Still the same the same. Or is it 220? I forget the exact number. It’s 220 or 250. Whatever. It remains the same all the way out to the outskirts of the galaxy, and so the galaxy is not a little solar system. It is something else, and you cannot – you just can’t get that rotation curve without 10 times the mass in the galaxy. That if there was – you know, you could see 10 times the mass in black holes all around the galaxy and they were visible somehow, then that would explain it.
Dr. Pamela Gay:
Yeah. It’s the equivalent amount of matter of taking one Acme brick per solar system sized volume in the outer galaxy. So, you can imagine just all the Acme bricks floating around, and the MACHO Project has gone looking for the universal version which is neutron stars, stellar mass, black holes, white dwarves, and hasn’t found them.
Fraser Cain:
Yeah, and so you take a person who doesn’t – who rolls their eyes at this, and you say, “Okay, fine. So how? How does this work? How do you get the stars not slowing down in their orbital velocity like you would see in a solar system?” And then the person has to say, “I don’t know.”
Right! Done! You now are part of the dark matter belief system, right? Weird observation. Why is this happening? I don’t know. Good enough. Join the club. Here’s your membership card. You’re now one of us, and so yeah. It’s called dark matter, but who knows what – it could be as you said, particles. It could be black holes, and it could be that we don’t understand gravity at the longest scales. Doesn’t matter. Still’s dark matter.
Dr. Pamela Gay:
And it can be a combination. I just want to make that clear.
Fraser Cain:
It is almost certainly a combination of all of them. And done. You are part of the confusion that nobody knows what this thing is, and yet you can – people can make these observations with relatively small telescopes. Vera Rubin did it in the – you know, almost a hundred years ago, and yet here we are, still arguing about what it is.
Dr. Pamela Gay:
And little, tiny radio telescopes that universities have allow us to go even further out in the galaxy than what Vera Rubin initially did with optical light. Because we can start seeing the neutral gas that is the furthest stuff out in our galaxy, and so, yeah grab yourself a small optical telescope and a small radio telescope, and you’re done. You can prove there has to be something invisible out there affecting the rotation curve.
Fraser Cain:
And there’s one last piece of spiral galaxies that I think is really important, which is the monster at the heart of them.
Dr. Pamela Gay:
And as recently as the 1990s, people were drawing on overhead sheets little, tiny monsters, usually with antennae, and giant mouths like vomiting jets out of the cores of spiral galaxies. So much has been lost now that professors aren’t hand drawing on overhead sheets. It’s truly a lost art, and we are suffering so many fewer cartoons as a result of it.
So, yeah spiral galaxies, there is a relationship, and this works for ellipticals as well. There’s a relationship between the size of the bulge and the size of the supermassive black hole in the center. There are some galaxies, like less than 10 last I looked, that look like quite maybe, possibly, could be they don’t actually have a supermassive black hole, and they don’t have a bulge but still working on it.
Fraser Cain:
Millions do. Ten don’t.
Dr. Pamela Gay:
Right. Exactly. And so, when you see these systems with large bulges in the core, with lots of high velocity stars in those bulges, they’re going to have the big supermassive black holes. Smaller bulge, lower motion, smaller supermassive black hole. And what’s neat is, depending on the angle that we’re able to look in on a supermassive black hole that’s feeding, we get all sorts of different cool effects.
So, if you have a system that’s edge on and has a supermassive black hole in the center, it’s called a Seyfert 2. They’re kinda boring. They don’t have very exciting lines that do very much, but they are active, and they show up in the radio in new and interesting ways. Now tilt that towards us, and you start to get what’s called a Seyfert 1. Tilt it straight towards us and give it a really powerful jet, and you start to get what’s called a blazar. And here, because the distance that it – the time that it takes light from the far jet to get to us and the time that it takes for light from the near jet to get to us, it gives the perspective of faster than light motion between the two ends of the jets, so there’s just really cool physics.
Fraser Cain:
Yeah, it’s really awesome. All right. Well, I think we’ve covered spiral galaxies, and so next week, we pick up the story with the giant elliptical galaxies. Thanks, Pamela.
Dr. Pamela Gay:
Thank you Fraser and thank you to all the folks out there that support us through Patreon. We really rely on you so very much. Beth pulled together putting these three episodes for us on a dime when I told her yesterday.
Fraser Cain:
Surprise!
Dr. Pamela Gay:
Yeah, “Guess what?” And Rich is out there doing all of the editing, hiding so many blunders. We thank you, Rich. Ally’s out there helping with our YouTube channel. It takes a team to make this happen. This week I want to thank Kimberly Reick, Disaterina, Jeff Willson, Tim Gerrish, Greg Wilde, John Drake, Robert Cordova, Paul D. Disney, Veronica Cure, Michelle Cullen, Phillip Walker, Benjamin Davies, Dwight Ilk, Brian Kilby, Daniel Loosli, Saebre Lark, Sydnie Walker, David Bogaty, Evilmelky, Justin S., Maxim Levet, Hal McKinney, BebopApocalypse – I love that one. Dianne Phillippon, Bruno Leitz, Ruben McCarthy, Larry Drozt, Bob Zatzke, Timelord Iroh, Frank Stuart, and Jason Kardorkus. Folks, you donated $10. We are grateful and this means I mispronounce your names. I’m sorry.
Fraser Cain:
You won. Thanks everyone, and we’ll see you next week.
Dr. Pamela Gay:
Bye-bye. Astronomy Cast is a joint product of Universe Today and The Planetary Science Institute. Astronomy Cast 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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