 Our second talk is also about galaxies, if we accept the broad definition of the last talk is about galaxies, and this one is actually more about galaxies themselves. It will be given to you by my colleague, the wonderful Grace Stumpford. Join me in welcoming Grace, who is a PhD student at the University of Washington. Tonight this is an awesome crowd. So, following up on Jen's talk, I am here to basically take you through all the other kinds of galaxies that exist in our universe. This is my research field. This is the stuff I'm really passionate about, so I'm really excited to share this with you. So, I say a whirlwind tour of galaxies because people teach entire classes about this, and I'm going to hit the highlight reel in the next 20 minutes or so. So, get ready. So, before I even start my talk, I want to point out this image that I have here. I have galaxified my title slide using writing.galaxyzoo.org. So, this is actually generated using images from the Sloan Digital Sky Survey. People have found these images using GalaxyZoo, which is a citizen science tool, and I wanted to advertise this to you because I feel like if you're here with me on your Wednesday night drinking beer and hearing about galaxies, you might be into this. So, you can actually log into this website and help scientists achieve their science goals by classifying images. Because human eyes, let's face it, they're just better than computers. No matter what algorithms we write, sometimes it's just better to have humans look at stuff. So, you can classify galaxies based on their shapes and help people do really interesting science that leads to some of the results I'm going to share with you tonight, which is awesome. And galaxies aren't really your thing. You can classify all kinds of other stuff like planets and penguins and cyclones. There's tons of stuff on zoomers.org. So, here's just a plug for that. Check it out if you're interested. So, back to my actual talk. If you were a Mac owner circa 2011, this is going to look really familiar to you. This is your desktop app now. This is a galaxy in our galactic neighborhood. This is an M31 or Andromeda. This galaxy basically looks like the Milky Way. It's a little bit bigger, but otherwise it's basically the same thing. I'd rather bet that this image is basically what comes to your mind and mine actually when you hear the word galaxy. This is pretty typical. So, this galaxy is sitting in a big dark matter below like all galaxies are. This is just most of the mass in galaxies, but it doesn't actually emit any light. But it's there. This galaxy has a nice disk structure. It's got something like 100 billion stars. It's forming new stars, but at a pretty low and slow steady rate. So, this is like a pretty typical galaxy for our part of the universe. So, my job tonight is to basically show you all the things that are galaxies that don't really look like this. So, to do this, I'm going to go through a bunch of different ways that we classify galaxies and a bunch of different categories that we put galaxies in. I'm going to show you the span of galaxies across each of these categories. So, first up, we have the stellar mass or brightness of the galaxy. Basically, the more stars the galaxy has, the brighter it is. So, we'll see galaxies cover a wide range in these properties. We have morphology or the shape of galaxies. So, you see my very elegant spiral arms there, or they can look pretty much just like blobs. We also have the star formation rate. So, how quickly galaxies are building up new stars on top of the ones they already have. And we have something called nuclear activity. So, most galaxies host black holes, supermassive black holes at their centers. And basically, sometimes these black holes eat a lot of gas really quickly and then they blow out a whole bunch of energy. And so, that's what these little jets are. That's nuclear activity. We'll circle back to those later. Okay. And feel free to interrupt through questions, by the way. Throwing that out there. Galaxies cover a huge range in stellar mass. They are not all basically the same size by lock shot. So, we have our Milky Way-like typical galaxy here. These galaxies, they're fairly common. Basically, if you try to make galaxies much bigger than that, they become a lot less common. It turns out it's really hard to make galaxies that are much bigger than our Milky Way. So, at the very, very massive end, you have galaxies that are something like 10 times more massive in terms of their stellar content. And at the other... Sorry, I didn't hear you. That wasn't a question. Okay. So, at the other extreme, you have these dwarf galaxies, which are extremely common relative to more massive galaxies. And a dwarf galaxy is something that is at least 100 times less massive than our Milky Way. They can be much more or much less massive than that, actually. So, we could really draw this line way off this plot, but that wouldn't be a very interesting plot to look at. So, dwarf galaxies, here's your general picture. And I just want to emphasize that this is just kind of the most well understood part of the galaxy population. And this is, you know, a thousand times different in stellar mass. So, this is a pretty wide range we're talking about here. So, at the very, very small extreme end, we have ultra-faint dwarf galaxies. In this image I'm showing you, there is a galaxy called Boots 1. Does anybody think they can point to that galaxy? Tell me where to put my laser pointer. Up? Up here? Down? It's actually filling basically this entire image. You just can't see the contours outside of it with your naked eye because there are so few stars in this galaxy, you cannot pick it out by eye. This is a satellite galaxy of our Milky Way and all ultra-faint dwarf galaxies that we know of are Milky Way satellites because they are so faint, you literally cannot detect them far away from our galaxy. So, essentially what astronomers have to do is measure the individual stars in this image. Do some kind of modeling to figure out which ones belong to our galaxy that are actually in the foreground. Get rid of those and then pick out just the stars that belong to this ultra-faint dwarf and do science with them. So, these are very, very tricky little tiny galaxies. By this planet galaxy? What I just said, basically you measure all the stars and you model out the ones that are in the foreground. I mean, really, that's the only way you can do it because you can't just say by eye, oh, there's a galaxy in this image. You need to use detailed measurements of the stars to figure out which ones are part of the dwarf and which ones are part of the Milky Way. Does that make sense? Yeah. Okay. So, this galaxy has a brightness that's something like 10,000 times the brightness of our sun. For reference, that is something like a million times fainter than our Milky Way galaxy. So, this thing is really, really main note. This is just kind of a fun oddball class of galaxies. These are ultra-diffuse galaxies. So, you can kind of see the galaxy in this image. It's the sort of fuzzy blob in the middle. So, that is the galaxy. This is in a galaxy cluster and essentially it is the same physical size, you know, diameter across as our Milky Way galaxy, but it has 100 times fewer stars. So, it's much, much less massive and it's actually basically held together gravitationally by its dark matter. So, these galaxies are actually a pretty recent discovery. The paper that I took this from came out in 2015 and this was sort of the discovery paper. So, this is kind of an open area of research. It's pretty hard to explain how to form these weirdo galaxies that are not very massive at all, but huge. All right. And then at the very, very large extreme of the galaxy spectrum, I suppose, we have brightest cluster galaxies. So, this image is of galaxy cluster. Basically, there is a ton of mass in this structure. It's mostly dark matter and the amount of matter here is so great that it's bending the light from the galaxies in the background. So, that's what these streaks are in the image. It's basically distorted light that's caused by space actually being bent by the mass that's in this galaxy cluster. And so, this galaxy in the middle is the brightest cluster galaxy and it's essentially been sitting there at the middle of this huge galaxy cluster eating up other galaxies as they make their way into the center. And so, through this process of merging with smaller galaxies, it can become very, very massive in terms of stellar content. So, this is basically how you form the very, very brightest and biggest galaxies in the universe. So, there is definitely a supermassive black hole at the center of that galaxy. And as that galaxy eats smaller galaxies, their supermassive black holes will also spiral it. That was almost certainly a Hubble Space Telescope image. Okay. So, galaxies do not just, you know, perform and then stay as they are. I'm sort of alluded to this, but galaxies are doing things. They are constantly changing. So, the basic thing that galaxies do is accrete gas and turn that gas into stars. And by accretion, I mean there's gas in between galaxies. We call this the intergalactic medium. And basically, this gas will flow into galaxies along these sort of filament-like structures and kind of settle into the middle of the galaxy. And once it's there, it can continue to condense and eventually form stars. So, that's basically what galaxies are there, star-making machines. And as this star formation progresses, the young stars will explode as supernovae and they inject a whole bunch of energy back into the gas within the galaxy and essentially drive a lot of that gas back out. So, galaxies are surprisingly dynamical places. And all these processes of star formation and the associated gas flows, this will continue as long as there is this gas supply available to fuel star formation. We actually see a lot of galaxies in the universe that are quenched, which is astronomers speak for no longer forming stars. We also call them red and dead, and we'll circle back to this in a moment. Okay. So, this leads me into the idea of galaxy morphology. It's just the shape of a galaxy. This is the basic spread of galaxy shapes that we see. Over here we have elliptical galaxies, designated with ease, very creatively. And over here we have spiral galaxies, designated as assets. This top branch is just regular old spirals with sort of a bulge component and the bottom branch here has bar structures. And then over here we have irregular galaxies, which are exactly that. They just look kind of irregular. They don't look like other galaxies, and so they get their own special category over here. So, you might recognize this name of the diagram, the Hubble tuning fork. Same name as the very famous telescope, but the diagram was not named for the telescope. It was named for the man, Edwin Hubble, who came up with this diagram as part of his sort of first theory of, you know, galaxy evolution back in 1926. So, this is a very old idea. So, there's this idea that some evolution happened. One side of this diagram turns into the other side of the diagram. Which way do you think the direction of evolution goes? Are we going from left to right or from right to left? Right to left. Right to left. Yes. Yes. So, this is true. This is what we understand to be true today. However, back in the day, when Hubble first made this diagram, he thought that it went from left to right. And because astronomers really hate giving up our historical jargon, to this day, these galaxies are referred to as early tight galaxies, and these are late tight galaxies. I'm not kidding it off. This is the jargon. Wait! So, modern understanding is that generally speaking, things on the right of this diagram turn into things on the left of this diagram. So, things over here are blue galaxies, which you can see just by looking at them. The blue light is due to the sort of blue or light that is output by young, short-lived stars. So, if you don't have recently formed stars, your galaxy is not going to look blue. They have a lot of gas, and they're still forming their stars. Over here, the elliptical galaxies in general tend to be pretty massive. They tend to have this red color, because they don't have any new stars, because they are quenched. So, okay. We have blue, and we have red. Think about the rainbow. To be very, very small galaxies, they're typically dwarf galaxies, and so they're still forming stars. They will probably sort of join the sequence at some point later, as they continue to build up. They're at a very early evolutionary stage. It's hard to tell. You can't predict the future, I guess. So, we have our blue galaxies and our red galaxies, but we're missing orange, yellow, and green, right? So, where are those other colors? So, this transition happened, but it's really unclear why. It's still an active area of research. People have a lot of good ideas, but it's not a solved mystery, and there are probably a lot of different pathways for this to happen, so just keep that in mind. I'm very clear about the generic statements, but in detail, this is probably not how everything works in a very smooth way. So, in between the red and the blue, where are the green galaxies? So, another way to think about this color dichotomy is in terms of the amount of star formation that's going on. So, most galaxies in the universe live in one of these two clumps. We have the quenched red galaxies down here, not forming a lot of stars, but a sequence of bluer galaxies that are still forming their stars. But there are some galaxies populating other parts of this diagram. We do, in fact, have some green galaxies, although they don't look green. If you find a picture of a galaxy, it doesn't actually look green to your eye. We just agree because it's in between red and blue. And then we also have these starburst galaxies up here. And so, there are very few galaxies in these two parts of the diagram. We basically call these transient populations because they don't really stick around in these parts of the diagram for too long. And I will talk a little bit about both of these categories right now. So, first up, we have our starburst galaxies. So, these are star-forming galaxies that have somehow recently come into a whole bunch of gas that they didn't have available to them before. And so, they have the ability to form stars at a very, very high rate relative to sort of the normal amount of star formation for a galaxy of its size. And as a result, you have a lot of these massive young stars that are dying and exploding as super-node and injecting a lot of energy into the gas. And so, this image is showing you these outflows of gas coming out of the galaxy because there's just so much star formation happening that it can essentially evacuate the gas from the galaxy. And so, these objects are short-lived and they do not hang out in the starburst area of that diagram because they essentially exhaust their gas supply very, very quickly and will fall back down to sort of more normal star formation rates. Yeah. Could you describe the time frame you're talking about? Yeah. Okay. Great point. Yes. So, short in terms of astronomical time scales is something like 100 million years up to a billion years. Yeah. You can tell I'm spending too much time thinking about galaxies. This is my concept for short. Galaxy galaxies, which we do think are a transition population. So, what I'm showing you here is basically that same diagram except the real data, but this time it's flipped. So, now things at the bottom are forming fewer stars. I'm sorry. Things at the bottom are forming a lot of stars. Things at the top are not forming stars. And this is basically a contour map showing the number of galaxies. So, there are a lot of galaxies right here and a lot of galaxies right here and not very much in between. And so, we have our red galaxies up here, blue here, not much in what we call the green valley. And the idea is that galaxies are hanging out in the star forming portion of the diagram, but then they quench for some reason and they move relatively quickly, again in astronomy terms, a few hundred million years to a gap year, move relatively quickly through the green valley and make their way over to this quench population. And so that's the explanation for why we don't see a lot of galaxies in that part of the diagram. Things just move through there relatively quickly. So, there are probably a lot of mechanisms at play in that transition and it's probably different for different kinds of galaxies, but one really promising idea about how this works is at least for some galaxies, an active galactic nucleus probably plays a big role in the quenching process. So, I briefly referenced this in the beginning just to refresh your memory. Most galaxies have a supermassive black hole at the center. If there is a gas supply available to that black hole, the gas will accrete onto the black hole and it turns out that this process just produces a lot of energy. And so these jets form, a lot of energy is injected back into the galaxy and things happen, like these huge winds that you see here. And just to make it very clear, this black hole is tiny. The accretion disk is tiny, so on the scale of the galaxy, all of this energetic stuff is happening at the very, very center. But we think that these objects still have a really big effect across the entire galaxy. It's not just affecting things at the very center. And so it turns out that AGM-hosting galaxies are preferentially green valley galaxies. Again, this is not a cut and dry thing. We do see AGM in both red and blue galaxies, but preferentially they hang out in the green valley. So this is the general story that kind of ties together this idea of active galactic nuclei and starburst galaxies and quenching all in one neat picture. So for at least some kinds of galaxies, this is probably what's going on. So we have a merger between two, you know, normal star-forming gas-rich galaxies. This merger basically causes gas to funnel to the center of the galaxy, feed the black hole, and simultaneously feed star formation, the same gas sources both of these processes. And all of this produces a lot of feedback which essentially makes the gas no longer available for star formation, causes the galaxy to quench, and then you're left with your transition green valley galaxy and then eventually a quenched elliptical galaxy. So this is the broad picture of how quenching works. And best for last, of course, we have galaxy mergers. So this makes it very clear sort of how dramatic these big events are. So these are major mergers, and major means that the two galaxies that are emerging are kind of similar in their stellar mass. So rather than just being kind of a boring little galaxy being absorbed into a bigger one, you get these huge like cosmic train wreck kind of mergers. And so basically the gravitational forces are huge. You get stars being flung out of the galaxies. These same gravitational forces are causing gas to get funneled into the center and sort of compressed, and it triggers star formation. So in this image here, all the pink is tracing ongoing recent star formation. So mergers are just fantastic things to have images of. You have Hubble Space Telescope again. And just to bring this closer to home, our galaxy is definitely going to undergo a merger like this in about four billion years. So we have our friend Andromeda, a member from one of the very first slides. Andromeda looks like this roughly in the sky now. As time goes on, it's going to start looking bigger and learning bigger, and then that's going to happen when our two galaxies finally collide and group each other apart. And then they're going to go through the process of kind of settling down and then being a really boring, quench-dred elliptical galaxy. So we have that to look forward to, but as Jen told us in her talk, the Earth is going to be roasted by then by our expanding sun. So we want you to be here to witness this epic night scene. So I will leave you on this very happy note. Thank you all so much. We have time for a bunch of questions. So let's say you did this with this, and we think it destroyed you. So the question is, let's say we were able to find another planet or another solar system to go and have it before our planet is killed. Would we be able to survive this event? That is, are all solar systems going to be destroyed when this kind of merger happens? And the answer is, we would probably be fine. As long as we found a planet that was far enough away from the star to survive the effects of the stellar revolution, and a happy place to hang out, individual stars are not going to be destroyed by this. There's actually a lot of space between stars, so the probability of star-star collisions is actually quite small. I don't know as much about, you know, potential gravitational effects winning planets out of solar systems, but that seems kind of unlikely to me. I think you'd probably be fine. Yeah, yeah. So the question is, what makes these really diffuse galaxies a galaxy as opposed to just a random collection of stars hanging out somewhere? So the basic idea is that these stars are all living in a dark matter halo, and they're all gravitationally bound together. So that's really the definition of a galaxy. It's a gravitationally bound system of stars, gas, dark matter, dust, things like that. So basically, based on measuring how the stars are moving, you can figure out how much mass must be present in that galaxy, and it turns out that for those very ultra-fanged works, they're heavily dominated by the dark matter component. So that's what's keeping everything together, and that's what makes it a galaxy. Any other questions? So if you asked me about the timeline between these images, I didn't super do my homework. I added the slide at the last minute. I don't actually know. Sorry. And honesty here. Yeah, I made a confusing schematic. Okay, so the question was about sort of my AGM picture, right? Like there's some red stuff going on in here and some blue stuff. That's just, I was picking the color scheme. So no, you can actually measure a galaxy color. It's the thing that you can measure. Basically, you take the brightness of a galaxy and a given filter, which basically selects certain colors. So you have a red filter and a blue filter and a green filter, and you can take the differences between brightness and those different filters. And that's the definition of a color in an astronomy perspective. So basically, you can make those color measurements, put them on a plot like this, and kind of see a natural division of curves between the red and the yellow. Great question. So the question is, are there any grown stars that aren't part of galaxies? I'm not going to say definitely not. I know that stars get ejected from star clusters and small groups of stars pretty often. We call these runaway stars. Basically, you have, you know, very close gravitational interactions between a few stars. You can get big kicks that grow a star out of that system. I think it would be pretty hard to get a star moving fast enough to escape the gravitational pull of the host galaxy, but I don't know, it's possible. So the question is about how some galaxies we know have higher concentrations of dark matter. And the question is, is that just because the map works out or how do we actually know? So basically, it is to a certain extent because the map works out because that's what a lot of science is, right? We take our observations, we do some math, we measure some stuff. But it's because we can measure the motions of stars and gas within these systems, and we can essentially model how much mass needs to be present within the radius at which we're measuring these motions in order to account for them. So basically, you can count up all the stars that you see and then you can figure out what mass needs to be there in order to explain the motion that you're observing and based on the difference between those two, you can figure out how much dark matter has to be there. So I don't think... Awesome. Yeah. The question is, if there's so much empty space in galaxies in between the stars and such, why don't they just pass through each other? Why do they undergo these murders? This is a very good question. It's essentially the gravitational interactions between the dark matter halos of the two galaxies, and the stars are affected also by gravitational interactions between individual stars. They don't collide into each other, but they do sort of interact. Is that one of the basis for the theory of dark matter? Possibly. So the theory of dark matter, with the first observational evidence of dark matter came from someone making measurements of the rotation of galaxies and how much mass you would need within a certain radius in order to account for that rotational motion. I think that observations of, say, galaxy cluster mergers support the evidence, or, sorry, support the theory of dark matter that we have. Is that answer your question? The question is, do all black holes form in galaxies? I'm going to venture, I guess, just because I think... Okay, so black hole formation itself is a whole tricky area of research that I don't know super long about, but my understanding is that you can form supermassive black holes by direct collapse, which means that you have a very high density of gas, and you can only really get that if it's concentrated kind of at the center of a dark matter halo. So I feel like it would be really difficult to get that happening, just kind of out in a diffused part of the galaxy, and not in the middle of a dark matter halo, which is at the center of the galaxy. Okay, so the question is, essentially, can there exist, like, a galaxy made of galaxies? So could you see sort of a spiral structure where instead of stars making up the spirals, you have galaxies making up the spirals then? Definitely. Okay. So, we haven't observed a spiral made of galaxies, but I will say that things are what we call self-similar, which means they kind of look the same on bigger and bigger scales. So you can have a relatively low mass galaxy that has a lot of galaxy satellites around it, a lot of smaller dark matter halos, kind of gravitationally valid, and when you go to a bigger object, like, say, a galaxy cluster, things basically look the same in terms of where the mass is distributed, but the only difference is instead of having, like, one real galaxy and a bunch of kind of teeny tiny diffused things, now you have a huge bright cluster galaxy and a whole bunch of, like, pretty mass galaxies in the back end. So, in a sense, yeah, things are kind of scale invariant, but not quite spiral made of galaxies. Okay, so the question is, I've been saying dark matter payload this whole time, what causes the dark matter to be plung to the outside of the halo, or is it dispersed all throughout? To the outside of the galaxy, I guess. So it is all dispersed throughout. It's actually concentrated in the center and kind of increases intensity as you go out to the side. It's really just, again, one of those stronger, drunken things. We like all the dark matter payloads, so they are without thinking. So it's not the case that there's sort of dark matter concentrated only around the outside and not in the middle. There's actually a lot of dark matter in here. Yeah. Okay, so the question is, what makes galaxies blank? So, essentially, it's the angular momentum of the gas that's accreted onto the galaxy. So, in this picture, you can see there's kind of like a net rotation that's happening here. So, as gas is accreted from the inter-blacked medium, it maintains this angular momentum. It's conserved. And so, the gas essentially settles down into a disk. And since gas is the fuel for star formation, star formation all occurs within that disk. So, it's kind of just a natural consequence of the angular momentum. Okay, so the question is, what is the primary driver for the color shift? Is it the Doppler shift or something else? So, that's a really good question. So, okay, what you're thinking of with the Doppler shift is what we in the galaxy community call redshift. That is essentially the shift in features in the galaxy spectrum that's caused by the net motion of the galaxy away from us as the universe expands. So, that is not quite the same thing as the colors I'm talking about here. The galaxies that I've primarily talked about are in the local universe, so they don't actually have a very big redshift. I get that it's confusing because the terms are very similar, but it's actually a different concept. So, essentially, when we observe galaxies, they're in a very distant universe. If we look at them through, say, a red filter, in terms of the wavelength that we observe here on Earth, what we're seeing is a light that was emitted in a blue work filter from that galaxy, and as it came toward us, the redshift that we expanded across the light to stretch the wavelength, which made it look redder than it was when we emitted it. So, that's kind of the connection. Two different things. The question is, what is going to happen to our galaxy and galaxy clusters as the universe continues to expand? So, yes, the universe is expanding, but structures like galaxy clusters and groups of galaxies are gravitational bound, which means that they're not going to be ripped apart by the expansion of the universe. That is a weaker thing. And so, essentially, the space in between galaxy clusters will continue to increase as the universe progresses in time, but within the gravitationally bound cluster, things will still be gravitationally bound. Those will get ripped apart by the expansion of the universe, because the global gravity dominates over that expansion. Let's thank Grace once more. We'll see you next month, November 14th, on Wednesday. Back here at Tebbler. Thanks for coming.