Ep. 682: Ultra-Diffuse Galaxies and Dark Matter

Astronomers first noticed the strange behaviors of rotating galaxies almost 100 years ago, suggesting there’s an invisible dark matter hold them together with gravity. Or maybe we just don’t understand how gravity works at the largest scales. Observations are much better now, and astronomers have found examples of galaxies that almost entirely made of dark matter. Does this tell us anything?


(This is an automatically generated transcript)

Fraser Cain [00:01:49] Astronomy Cast episode 682 Ultra Diffuse Galaxies and Dark Matter. Welcome to Astronomy Cast for weekly fact based journey to the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Piers are keen. I’m the publisher of Universe Today. With me, as always, is Doctor Pamela Gay, a senior scientist for the Planetary Science Institute and the director of Cosmos Quest. How are you doing? 

Pamela Gay [00:02:12] I am doing well, and I have exciting news. 

Fraser Cain [00:02:15] I’m ready. 

Pamela Gay [00:02:16] We. We have started releasing Escape Velocity Space News as a podcast. So if you’re not into the hole sitting down in front of a TV and watching stuff on YouTube, now you can just listen to us while you do whatever it is you’re doing. So check it out wherever podcasts are found. 

Fraser Cain [00:02:36] So search for Escape Velocity podcast and. 

Pamela Gay [00:02:40] Is sadly, there’s a bunch of us. 

Fraser Cain [00:02:42] There’s a bunch of escape velocity. So Escape Velocity Space news podcast featuring Doctor Pamela Gay. And for. 

Pamela Gay [00:02:48] Us. 

Fraser Cain [00:02:48] It’s okay. There you go. And how often are you releasing episodes? 

Pamela Gay [00:02:52] So we have a back catalog, and right now we’re releasing about two week as we get through the back catalog, and then it will be one a week in the future. 

Fraser Cain [00:03:00] And how long are they? 

Pamela Gay [00:03:02] They’re about, 40 to 50 minutes. 

Fraser Cain [00:03:05] Okay, great. Astronomers first noticed the strange behaviors of rotating galaxies almost 100 years ago, suggesting there’s an invisible dark matter holding them together with gravity. Or maybe we just don’t understand how gravity works at the larger scales. Observations are much better now, and astronomers have found examples of galaxies that are almost entirely made of dark matter. Does this tell us anything? All right. Hey, what’s dark matter? 

Pamela Gay [00:03:35] It is stuff. Okay. There’s stuff that does not like to interact with the electromagnetic force. No light. 

Fraser Cain [00:03:42] But, like, no conclusive answer. 

Pamela Gay [00:03:45] It’s stuff. Stuff on my my personal. My personal feeling is it’s going to be a mix of science and sterile neutrinos that get us to the correct temperature profile. 

Fraser Cain [00:03:58] Oh, we should look. I mean, I think the answer, based on the conversations that I’ve had with astronomers so far, is, I mean, I think you’re on the right track there. It’s going to be a whole bunch of things. It’s going to be it could it’s a few primordial black holes, a few rogue planets, a few, axions. 

Pamela Gay [00:04:18] Some a lot. 

Fraser Cain [00:04:19] Of things that we don’t see. 

Pamela Gay [00:04:21] A lot of vaccine. 

Fraser Cain [00:04:22] A lot of action. 

Pamela Gay [00:04:23] To take. 

Fraser Cain [00:04:24] Neutrinos some. Right. Like just a whole bunch of stuff. So we don’t really know. Yeah. But. What is it? You know, even though we still don’t know, like, literally, we could go back and talk to Fritz Zwicky in 1930 and say like, what is it? You know, and he would be proposing the same kinds of things that we’re talking about today. The key, I think, is that astronomers have gotten so much better at mapping and observing it so well. What is the modern state of of observing dark matter? 

Pamela Gay [00:05:02] So there’s there’s two primary approaches. One is to look at how dark matter changes our ability to see the universe behind it. Matter in general has this wonderful attribute of being able to bend light with its gravity. Dark matter is matter. And as as light from background galaxies shines towards us. Dark matter has the capacity to change how the light moves through the universe. And what’s really cool is we expect on average, that if you look at a swath of sky, all the different galaxies with all their different edge on, face on twisted, all their different shapes should average out to a desk. And when we move our swath across the sky, what we see is over here you have teardrops. Over here you have Christmas trees as as the average shape in those swaths. And those nine round shapes tell us the distribution of dark matter that is changing how these background galaxies look. And we can actually start to do it in different distances where we look at galaxies super far away. How are they distorted? Look at the middle distance. How are they distorted? Look at the closer by how are they distorted? And this gives us a three dimensional map of of dark matter on the largest scales. And then on smaller scales we can look at things like the bullet cluster. We have two clusters of galaxies slamming together, and you can map out the dark matter in that system by looking at everything behind it. So we use gravity. Grab this image. The only way. 

Fraser Cain [00:06:54] And these maps at this point are quite comprehensive. Like, yes, they have observed huge swaths of the sky, mapped out the distortions and use that to reverse engineer where the blobs of extra gravity are. And there are some wonderful, like 3D visualizations of these, and they really look like kind of blobs that surround. So if you look out and you see the galaxies, the giant galaxy clusters, then you see them in these lines and walls and structures like that, you if you can see the dark matter, just everything would be surrounded by big blobs of some kind of invisible stuff. And, and that’s the part that really amazes me is, you know, we don’t know what it is, but we know how to use it as a telescope is the exact joke that I always make. 

Pamela Gay [00:07:46] And and it’s it’s so frustrating that particle physics requires us to understand the universe at the tiniest and the largest scales, and getting in both directions simultaneously requires twice the equipment. We’ll get there eventually. We’ll get there eventually. 

Fraser Cain [00:08:07] And so here we are in May 2023. Yes. And what is the state of the art? Like, what stuff can we dispose of? What ideas can we throw out, and what ideas remain about about what this is like? How well have we at least constrained the problem? 

Pamela Gay [00:08:28] So so the other way that we were getting at the distribution of dark matter is to look at how galaxies rotate as a function of distance out. And one of the thoughts was galaxies might just be full of dark material. There might be a bunch of super cold white dwarfs hanging out, being very old and not eliminated. There could be a ton of black holes, stellar mass, and smaller out there hanging out darkly. And all of these not giving off a lot of light things that we’re just not noticing could add up to be the dark matter that we need. It turns out that roughly the amount of dark matter necessary to explain what we observe could be accounted for if you took one Acme brick. And let’s face it, Acme bricks don’t give off a lot of light and put it in every solar system sized volume of space, and all those bricks would add up to what we need. So astronomers went out and looked for the gravitational lensing of individual background stars that we would see if, like a black hole passed in front of the background star, if a cold white dwarf passed in front of the background star, and we saw plenty of things get gravitationally lensed, but we didn’t see enough to add up to be all of the dark matter that we expected. 

Fraser Cain [00:09:57] Right? Right. And I mean, have we been able to at least rule out some stuff? 

Pamela Gay [00:10:03] Yes. So we have ruled out large populations of white dwarfs that are called cold neutron stars and rogue stellar mass and smaller black holes. 

Fraser Cain [00:10:14] I recall like the like if is black holes, if if dark matter is black, hold them. They either have to be greater than a thousand times the mass of the sun, or less than the mass of an asteroid. Right? You can’t have anything in between. Like those have all been ruled out, and similar things have been ruled out for particles, for various types of particles, various behaviors of particles, various. And then in particle accelerators like CERN, they’ve been able to rule out masses of particles that could account for dark matter. 

Pamela Gay [00:10:44] So we keep not creating weakly interacting massive particles. Particle physics. Basically, you take two particles, slam them together with vast amounts of kinetic energy, and the combined kinetic energy and mass energy turns into new particles. We also use giant tanks that are looking at cosmic rays and other particles, to see how they interact with the material inside of these vast tanks, all of these different ways of looking for dark matter, neutrinos, other things that are just very temporarily around. We we look for in these ways. And supersymmetry hasn’t been proven true. That entire second family of particles that we were expecting, not their weakly interacting massive particles not there. And as we look more and more at what was necessary to get the cosmology that we see, the rate at which galaxies formed, the amount of galaxies and superclusters and large scale structure that we have today. You also need particles that have a certain we say temperature. That just means how fast they’re moving and weakly interacting. Massive particles don’t have the right temperature profile to be the kinds of stuff that we were looking for, right? 

Fraser Cain [00:12:18] So like the hot neutrinos, the things streaming from the sun moving at close to the speed of light, they wouldn’t clump, remain, and clump around galaxies in the way that dark matter seems to do. So we can rule out these fast moving particles. So whatever it is, it has to be slow. 

Pamela Gay [00:12:39] And but not too slow. 

Fraser Cain [00:12:40] Not too slow. And it has to interact almost never because. Yeah, larger and larger tanks of why the kinds of things that have found neutrinos have failed to find these dark matter particles. And yet like. I’m like whenever I post videos about this on on YouTube and stuff, I just get tons and tons of comments of people rolling their eyes and going lol, scientists making up stuff again. But like. If you made the same observations that astronomers did, you would come to the same conclusions. Yeah, that that this stuff is there, that we just don’t know what it is. And and you have a, you have a whole series of wonderful analogies to sort of go into the mysteries. 

Pamela Gay [00:13:31] This is the stuff I just love. And with dark Matter, I had one very stark moment in graduate school. I, we had just had a seminar speaker that was talking about modified Newtonian dynamics, and I walked into one of my dissertation advisor’s offices and he his hands over eyes, glasses off, and I’m like, are you okay? And his response was something along the lines of, I think it’s true. I think there could be modified Newtonian dynamics, because at that point in time, we didn’t have all the observational evidence that we have today for the distribution. And the frustrating thing is, even today, there could also be this extra term to gravity. Yeah. So so we’re in this situation where we know the universe likes to improv. We know that it’s all about both. 

Fraser Cain [00:14:34] And yeah, yeah, I like I wish I could like you say to the person okay. Take a powerful telescope and observe the rotation rate of a galaxy. Okay, great. It’s a straightforward process to do. You measure the redshift and the blue shift depending on which side of the galaxy that you’re looking at. Okay, great. Now count up the total mass in the galaxy and work out how fast the galaxy should be rotating to tear itself apart. It should have torn itself apart. It doesn’t. Can we can we agree on this? Okay, great. Why? That’s it. That’s like. That is the beginning of the question of what is dark matter? Why? Why is the galaxy tearing itself apart? Either has an invisible mass or we don’t understand gravity. 

Pamela Gay [00:15:29] And then we get to the nominal topic of today’s show, which is, yes, an ultra diffuse galaxy is a galaxy that has 10 to 100 times fewer stars and pretty much no gas when compared to the Milky Way, but is spread out over the same volume. So you literally take. Part of a galaxy and go swamp and just spread it out and stupid of gas, and then try and find other similarities and realize there are none. They’re all different and they’re weird. And what’s really awesome is are examples of galaxies that have absolutely no dark matter, where the rate at which stars are moving in the system correlates directly to how much visible stuff is in the system. Those are ultra diffuse galaxies. And at the same time, some of the systems that we look at that have the most dark matter, also ultra diffuse galaxies. 

Fraser Cain [00:16:38] And so we’ve got like two example or we have like two flavors of extreme here we have galaxies that are that have enormous amounts of dark matter and very few stars. So when you look at it and you use it as a lens. It acts like a gravitational lens of a much more massive galaxy, and you can barely see the stars in it. And then you have the the other version of it, where it’s a very bright galaxy, and yet it acts like a gravitational lens of a galaxy that is far less massive than it is because it has very little dark matter in it. 

Pamela Gay [00:17:16] It’s never ultra bright. It’s just only as bright as its stars allow, right? 

Fraser Cain [00:17:21] Yeah, bright. In comparison to the old. 

Pamela Gay [00:17:25] On the mass. Yeah. So the mass to luminosity ratio is the way we usually talk about it. And, and these systems are consistently like 10 to 100 times smaller than the Milky Way in terms of luminous stuff. 

Fraser Cain [00:17:41] Right. And so. What makes this weird is that the amount of dark matter ratio to regular matter can even match. Yeah, and so you can have galaxies which have had their dark matter stripped away, and galaxies which have had their stars stripped away. When you have flavor in between. And then there’s like an average like on average, you’re going to get a galaxy matter and ten times as much dark matter. But you can get edge cases on both sides. 

Pamela Gay [00:18:10] Yes. 

Fraser Cain [00:18:11] Which is weird. And and I guess that’s a problem for the modified gravity idea. 

Pamela Gay [00:18:18] Right? It puts limits on it. It doesn’t rule it out completely. It just puts some limits on it. And that like I said, the universe is an improv artist. So what we’re beginning to think is true with these systems is they didn’t form the way we see them. They they’ve gone through things. They’ve seen things. It appears that if you trace back the motions of these systems, you can trace it back to some sort of a massive interaction in the past. These are potentially the splashes of galaxies given off during massive collisions. And in some cases, they take their dark matter with them and in some cases they don’t. And. It’s that sometimes they do, sometimes they don’t. That means dark matter is stuff yet again. 

Fraser Cain [00:19:20] And what is the mechanism that that could strip the dark matter out of a galaxy? 

Pamela Gay [00:19:27] We’re still figuring that out. I mean, it’s one of these things where we know that dark matter in general doesn’t have the same collisional cross-section that like an electron or a proton or any normal kind of matter that we’re made of would have. You throw electrons at each other and they’re willing to collide. You throw particles of dark matter against each other, and they really their collisions. For the most part. It’s like rolling VBS through a theater. Most of the babies won’t manage to hit a chair like, because they’re so small compared to the emptiness between the chair legs. Now. If you’re in one of these circumstances where the way that some particles just don’t seem to collide can cause the dark matter to pass through, along with some of the splashed out material. And now you have a dark matter rich scenario where in other cases, due to differences in the velocity of the collision and differences in how everything comes together, you can end up with the dark matter lingering behind while some of the splash carries on by itself. We’re still figuring it out. 

Fraser Cain [00:20:43] So what’s interesting about this research is that some of these ultra diffuse galaxies have been found relatively close to the Milky Way. So this is not the kind of thing that you would see shortly after the Big Bang. But in fact, we’re seeing them within our universe neighborhood. 

Pamela Gay [00:21:01] And what’s frustrating is we just don’t know when they started forming, because the whole ultra diffuse part means they’re super hard to find, right? And, and we’re starting to build news systems that are designed specifically to go looking for these kinds of low surface brightness things. But we have luckily been able to find a few of them relatively nearby. That gives us the chance to basically go spelunking through the data, looking for supermassive black holes, looking for details on the stellar motions and that nearby. This makes all these kinds of research questions possible. 

Fraser Cain [00:21:47] Yeah, yeah. I mean, do you want to talk about like specifically the research that was done? You know you give you know, you see gives some some new methods of research. Like what? What can you do when it’s that close. 

Pamela Gay [00:22:01] So when it’s that close, you can literally start going and looking at how fast are the different stars going? What are the stellar populations involved in this? Is there any gas you can start looking for? The 21 centimeter line of hydrogen to try and find any cold gas that might be there. And there is a new telescope system called dragonfly that is basically using a whole bunch of big old telephoto lenses to look extremely deeply into the region of the sky that was explored by the Sloan Digital Sky survey. They’re looking at that area because there’s already good background information, there’s already good calibration, and there’s spectra of pretty much all the bright stuff that Sloan could get spectra of. So they’re going into this knowing, okay, we’re finding the diffuse stuff that Sloan probably wasn’t able to see, but now we can put it into context. And this allows us to basically see how do we take normal galaxies and evolve them out to be what we’re seeing with these ultra diffuse systems? 

Fraser Cain [00:23:16] What do you think the future would hold for one of these galaxies? I mean, because they they don’t have a lot of gas and dust in them left a lot of stars. Yeah. Are they going to sort of fade away? 

Pamela Gay [00:23:28] Yeah. The these are these are systems that are very transient in nature. We’ve seen some of them that are far, far away from whatever cluster it was that shredded them. And so you have these broadly spread out collections of stars that are going to age away, fade away. They are going to have mass being given off through supernova and other activities, but it’s probably not going to be enough to re coalesce into new apex of star formation. They’re just going to fade from sight. And it’s sad and awesome all at the same time. 

Fraser Cain [00:24:15] Do you think we will crack this problem in our lifetime? 

Pamela Gay [00:24:18] Yes, it’s it’s one of these things where science advances at the rate of technology and instrumentation. We see with dragonfly the instrumentation. We need to find more of these systems that allow us to get more statistically rich understanding. We are also starting to get better and better computers that allow us to run the simulations. We have for a long time focused on how do we replicate the tadpole, how do we replicate the mice? When looking at how galaxy. 

Fraser Cain [00:24:54] Minerals are galaxies, by the way? 

Pamela Gay [00:24:55] Yeah. Yeah. Sorry. These are these are merging galaxy systems. 

Fraser Cain [00:24:59] The mice galaxy. Yeah yeah. 

Pamela Gay [00:25:00] Yeah yeah. Good. Very important clarification. 

Fraser Cain [00:25:03] Yeah, yeah. 

Pamela Gay [00:25:05] What what we haven’t been as focused on trying to understand is what happens to those tidal tails that get stripped off in the long run. What happens to this material that is cast off in the long run? With more computational power, we start to be able to understand the leftovers as well as the core material. And as we look at bigger and bigger systems in our simulations, as we look at more and more parts of our simulations, I suspect we’re going to see these ultra diffuse galaxies basically falling out as the splatter of these massive collisions. 

Fraser Cain [00:25:43] Yeah. I wonder if, you know, finding one of these relatively close because are so hard to observe. I wonder how much of the distribution of the, of the mass of the dark matter is coming from them. You know I don’t need an answer. I’m just. I’m just. I’m just wildly speculating, like I wonder. Yeah, if things like dragonfly or even Newt, you know, that idea of of better technology, of better ways to survey the surroundings that we might find. Okay, we actually see that that the dark matter is, is actually torn up into small pieces. And this is part of these little galaxies that would be really interesting as opposed to being necessarily like when you resolve a with enough resolution, you can start to see where that distribution of dark matter is more specifically than just hand wavy kind of around the galaxies. 

Pamela Gay [00:26:38] And dwarf galaxies in general are deeply confusing. Some of them will have a mass to light ratio in the hundreds because they are dark matter, extremely rich. Other systems will be in the tens and trying to understand what kind of evolutionary history goes into just your regular, everyday dwarf galaxy, having this range in possible mass to luminosity ratios, and then extending it out to bigger systems that have probably resulted from massive galaxies colliding. The physics is awesome and complicated, and I’m really glad I’m not writing those kinds of simulations. And I adore the work coming from the people who are writing the simulations. 

Fraser Cain [00:27:33] Wonderful. Good luck to them. 

Pamela Gay [00:27:36] Yeah, seriously. 

Fraser Cain [00:27:37] We’ll see you next week, Pamela. 

Pamela Gay [00:27:39] See you next week. And thank you to all of you out there who make our show possible through all of your donations. This week I would like to thank Jeremy Kirwin, Stuart Mills, Harold Varden Hogan, Claudia mastroianni, Daniel Loosely, Scott Beaver, Georgie Ivanov, David Gates, Scott Cohen, Jim Schooler, Justin Proctor, Marco Rossi, Matthew Horstman, Kimberly Reich, Alex Cohen, Tim Gerrish, Tim McMeekin. Cooper, Gregory Singleton, Consi pencil and co, Matthias Hayden, the big squish squash Jeff Wilson. Disaster. Anna and Paul de Disney. Thank you all so much for everything you do. 

Fraser Cain [00:28:26] Thanks, everyone. 

Pamela Gay [00:28:27] Bye bye. Astronomy cast is a joint product of the 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 Doctor Pamela Gay. You can get more information on today’s show topic on our website. Astronomy. Cars.com. This episode was brought to you. Thanks to our generous patrons on Patreon. If you want to help keep the show going, please consider joining our community at Patreon.com Slash Astronomy Cast. Not only do you help us pay our producers a fair wage, you will also get special access to content right in your inbox and invites to online events. We are so grateful to all of you who have joined our Patreon community already. Anyways, keep looking up. This has been Astronomy Cast.