The Milky Way has gobbled up dozens of dwarf galaxies and added them to its structure. Today we’re going to look at the ongoing hunt for the wreckage of past mergers. And what we’ve discovered about dwarf galaxies in general.
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What is a Dwarf Galaxy?
The Big Deal About Dwarf Galaxies
Dancing Dwarf Galaxies Deepen Dark Matter Mystery
CCD photometry of the Tucana dwarf galaxy
Surface Photometry of the Dwarf Elliptical Galaxies NGC 185 and NGC 205
Surface photometry of new nearby dwarf galaxies
Luminosity classifications of dwarf galaxies
Dwarf Galaxy/Dark Matter problem
Large and Small Magellenic Clouds are irregular dwarf galaxies
Shapes – Dwarf irregular, Dwarf elliptical and Dwarf spheroidal galaxies
Podcast Transcription provided by GMR Transcription
Fraser Cain: Astronomy Cast Episode 498: Dwarf Galaxy Update. Welcome to Astronomy Cast a 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, publisher of The Universe Today. With me, as always, Dr. Pamela Gay the director of Technology and Citizen Science at the Astronomical Society of the Pacific and the director of CosmoQuest. Hey Pamela, how are you doing?
Pamela Gay: I’m doing well. How are you doing, Fraser?
Fraser Cain: Good. So, we – I know people are sad, but we have warned them this is the final episode before we go onto our hiatus. So, of the season.
Pamela Gay: We scared people last year.
Fraser Cain: I did that on purpose. So, we’re going to be away for two months, July and August and then we will be back in September. And we’ve got some pretty exciting things that are gonna be happening in September. So, if you have been watching, paying attention, we’re all gonna be coming together in Edwardsville in September to your house to have a big party.
Pamela Gay: Not entirely at my house. We’re actually going to be spending September 15th at a brewpub most likely and then September 16th we are going to be at the Wildey Theater. So, yea, this is something you guys can get out and register for now. We’re going to have activities, maybe even fluid painting of tiny planets with me, Dr. Morgan Redburg is going to be joining us. And you can go register so that we can rent our spaces. So, to do that to register and for more information head over to Astronomycast.com, go to the trips pull-down menu, and click on AC500 weekend. Pre-registration is now open and requires a non-refundable $50.00 deposit.
Again, we need to rent the buildings. Everyone who pre-registers, even if you have to cancel will receive a goodie bag of memorabilia from the weekend. You may pay the full $200.00 registration fee now or you have the option of paying $50.00 now and paying the balance anytime between now and August 15th.
Fraser Cain: I can’t wait to see all of you there. And of course, during the summer if you’re feeling like you wanna still keep the spirit going, join the weekly space hangout crew, go to WHScrew.space. This is, of course, our fans, producers, our friends and they’re the organizers of this event, the people who help us get guest for the various shows, and sort of talk to each other about space while we are on our hiatus. So go to WSHcrew.space and they will hook you up. Alright, let’s get on with today’s show. So, the Milky Way has gobbled up dozens of galaxies and added them its structure. Today we’re going to look at the ongoing hunt for the wreckage of past mergers and what we’ve discovered about dwarf galaxies in general.
Dwarf galaxies, a very actual fascinating topic of tremendous sort of scientific discovery in the last 10 years. And they tell us a ton about both the evolution of the Milky Way and really the development of the large-scale structure of the universe itself. So, where do you want to start? What’s new? Or what are dwarf galaxies before we talk about what’s new with them?
Pamela Gay: That’s probably a good place to start and here I’m going to warn all of you my Master’s research was actually on dwarf galaxies, so this –
Fraser Cain: Dwarf galaxies nerd.
Pamela Gay: Yea, I admit to it. I admit to it. So, dwarf galaxies have been known for a long time. They are these incredibly diffuse, you can look right through them in many cases. Tiny galaxies that are located in the spiroid around our galaxy and around many other galaxies. There are some free-form ones just floating around, hanging out between the galaxies, but the places they really are found is in these halos of galaxies. Like globular clusters, the smallest among them formed all of their stars during a single epic of epic star formation. The earliest supernovas from this first generation of stars had such amazing shock waves that they actually were able to push all the remaining gas and dust out of these systems, leaving behind just lonely stars.
Fraser Cain: Oh.
Pamela Gay: Yea. So, they’re cute, they’re small in our galaxy, Andromeda, they’re found pretty much found the same disc as the rest of the stars and dust and nebula in our galaxy and Andromeda. And when we first recorded talking about these objects, we said things like, “They have the highest ratio of luminous to dark matter of any object that has been studied” and other things like that that are now entirely found to probably be untrue.
Fraser Cain: Oh, oops. So, can you go into a little more detail on that? What did we used to think?
Pamela Gay: So, it used to be that when we looked at these little tiny galaxies and studied the velocities of the stars within them, the velocities of what little gas we can see, we found that the velocities seem to correspond to these systems being made up of 99 percent dark matter and 1 percent regular, everyday stuff that we can see and detect. And what’s interesting is when we made these original assertions we didn’t screw up our observations, those were fine, but we based our calculations on the premise that let’s just calculate it for the dwarf galaxy only, completely forget that the dwarf galaxy is embedded in the halo of our own Milky Way galaxy and that was slightly problematic.
Fraser Cain: So, would you say that is – now you’ve moved into the things we’ve discovered, the things that are new, that we knew about dwarf galaxies?
Pamela Gay: Yes.
Fraser Cain: The biggest thing is just how many dwarf galaxies have been discovered here in the Milky Way. So, I don’t know if you know how many have been found or if you have that number in front of you –
Pamela Gay: No.
Fraser Cain: But lots?
Pamela Gay: Yes.
Fraser Cain: What is the technique that they’ve really pioneered and used to find all of these dwarf galaxies?
Pamela Gay: Well, largely it’s just counting stars that have similar composition and age. So, as you look out with instruments like – or surveys like the slim digital sky survey, Guya, all of these large area surveys that have very consistent photometry and any spectroscopy at all allow us to say hey over here there’s a whole bunch of stars that have these characteristics and look like they could of all come from the same parent population, but oh look they form a stripe across the sky. Or huh, we don’t have the ability to measure in detail the characteristics over there, but when we could the density of stars on the sky, what we find is there’s a stripe of over density where there’s a higher population than we would expect that is consistent with an arc of stars across the sky.
So, finding, well, shredded highly disrupted dwarf galaxies is now something of a sport for people working in these surveys. And I don’t think we can get past a major scientific conference without another press release on title stream from dwarf galaxy discovered.
Fraser Cain: Right. Absolutely, that’s one of the things, I mean, unfortunately, it’s getting to the point where it’s like we have planets discovered that we don’t even report them anymore. And I’m sure back in the beginning, a galaxy found shredded in the Milky Way is a pretty exciting story, but now it’s like meh, whatever. So, but those are the ones that are – the Milky Way has consumed in the past and of course in large and small magellanic clouds, there’s other dwarf galaxies that are around us and just part of the local group. So, have you learned anything new about some of these – not the ones that we’ve torn up and added to our mass?
Pamela Gay: So, our discoveries fall into two different scientific groups of information. The first one was we initially thought that it was unusual and some weird fluke that the majority of the dwarf galaxies going around our own Milky Way galaxy go around the disc of the galaxy. Andromeda has the same thing, we assumed that we were just weird because all of our models, all of our understanding of cosmology at this fine grains don’t have enough data level indicated hmmm this should be randomly distributed around galaxies. And as we’ve gotten better data that has allowed us to see dwarf galaxies forming and existing around other systems, we’ve come to realize that no they just tend to be in the plains of the galaxies they’re associated with and we can’t explain this.
Fraser Cain: Because you expect them to come in randomly from any direction not to be nicely aligned in the way they are. So, I mean does that mean they were spun out from the galaxies during the formation process or does that mean that they were –
Pamela Gay: They weren’t spun out, so that would imply that as our galaxy formed, as it became the nice discy spiral that it is globs of material got flung out like kids falling off of one of the spinny rides on the playground. And that isn’t quite what happened, but we have evidence in some colliding systems that as large galaxies collide you can end up with title tails of stars that over time with coilless into these dwarf galaxies. Now, it’s unclear if that is the origin of all dwarf galaxies, it’s most likely not, but it is the origin of some dwarf galaxies. And beyond that, we’re just kind of confused. Now, there are things that we can look towards, for instance, it has been found in massive studies of the orientation of galaxies, that galaxies that formed out of the same cloud of stuff tend to share these same orientations, whereas, when you look at galaxies as a whole they have randomized orientations.
It could be that these dwarf galaxies formed out of the same cloud of stuff as the massive galaxy formed. We still have more modeling to do. It’s still unclear and this is why we keep doing science to be able to answer questions like this.
Fraser Cain: Well and one of the big goals of the James Webb space telescope, of course, is to be able to look directly out into – to really almost to the edge of the observable universe to the time when these small dwarf galaxies were thought to be merging together into the large spiral structures we see today. So, what clues do we have right now about how these first building blocks were coming together back in the day?
Pamela Gay: We don’t really. This is one of the kind of awesome things about the dwarf galaxies is they are so poorly understood that it seems like a strange thing to find awesome, but they’re so fundamental and yet they’re so mysterious and it’s sort of like saying all of our food is made with flour and sugar, but no one knows where flour and sugar come from.
Fraser Cain: Right.
Pamela Gay: In this case, we have some people that theorize that dwarf galaxies are the left-over remnants of the cores of bigger galaxies that had the majority of the material stripped off into the formation of galaxies like our own and Andromeda. There are people that think no these are just the primordial galaxies, this is how everything started out and our Milky Way is made through the conglomeration of many, many of these tiny systems. We just don’t know. This is where telescopes like James Webb, when and if it ever launches, are gonna be so amazing because they can look back with sufficient resolution to the early days of our universe and start to tell us exactly how these things form.
Fraser Cain: So, there are a couple places I want to go. I want to talk about some of the classifications of dwarf galaxies that have started to come up, but one that’s recent news is this discovery of a dwarf galaxy that’s almost entirely dark matter. We’re you following this?
Pamela Gay: Yes, but I don’t know if we’re going to keep that result.
Fraser Cain: Oh, no. What?
Pamela Gay: So that was the other set of discoveries that I wanted to discuss. So, we initially thought that these things were, in general, pretty much all dark matter and the way we got at that result was by measuring the velocities of the stars in the system using the orbits of those stars to predicts the total mass in the system and then taking what can we see, that’s the luminous, take what we can’t see, that’s the dark, and you get your baryonic matter to your dark matter ratio this way. Now, when we did those calculations – I did not do these calculations, I have nothing to do with this. When people did these calculations –
Fraser Cain: When the calculations were done.
Pamela Gay: When the calculations were done, they were done without taking into any consideration the effects of the Milky Way.
Fraser Cain: Right.
Pamela Gay: And it turns out when you take in the effects of this giant galaxy sitting nearby, it kind of can lead to all of the motions that we see.
Fraser Cain: Right, but this galaxy, the one that I’m mentioning, this dark matter galaxy, was a little farther away, so it’s not one that’s that close to the Milky Way and it was found, I think it was by the motions of the stars in the galaxy that had the mass of a dwarf galaxy but seem to be comprised almost entirely of dark matter and the stars that were in there were very old. And it’s the kind of thing that was incredibly surprising, but not outside the realm of what you might expect to see with a big enough sample size of the universe. You’re gonna see every range, every ratio from no dark matter to lots of dark matter and they found one with the lots of dark matter – oh, they found one with no dark matter too, right?
Pamela Gay: I think that you’re talking about a galaxy that is normal galaxy massed, but has –
Fraser Cain: Oh, you’re right. You’re right. You’re right.
Pamela Gay: So, it’s with dwarf galaxies.
Fraser Cain: Yea, sorry. A regular galaxy – right, a sprialish galaxy found with almost no dark matter. Yea, apologies. Totally should save that for the dark matter episode.
Pamela Gay: No, it’s all good. It takes multiple brains to try to cook this up and put it together. So, it’s turning out that when you do your calculations and include the mass of the Milky Way you can get the observed velocities. Now, of course, there are people that are like well, maybe – and so people are trying to find multiple ways to prove is there, isn’t there with the amount of dark matter in dwarf galaxies. And one of them is there, isn’t there tests is to look at what happens to star clusters in the larger of the dwarf galaxies. So, while the smallest are just a few million stars that were all formed during a single epic of star formation, so there are things like the Ursa Minor dwarf spheroidal galaxy. There are other dwarf galaxies that actually have globular clusters and other interesting objects within them and orbiting around them.
But studying the orbit of a star cluster around a dwarf galaxy and doing camumatical simulations of well if you had enough dark matter to account for what we see, what would the effects on the star cluster be? If there’s a really large distribution of dark matter, that’s going to systematically destroy a star cluster of long periods of time. If instead, what you’re seeing is disruption caused braticationally by our Milky Way, that star cluster gets to survive and actually grow over time. And guess which one of the simulations actually matches what we actually see?
Fraser Cain: The second one?
Pamela Gay: Yea, the one with no dark matter.
Fraser Cain: Are you like starting to not be a fan of dark matter as its role to the formation and role in dwarf galaxies?
Pamela Gay: The role of dark matter in dwarf galaxies I think we completely screwed up and this delights me to no end.
Fraser Cain: See, people say all astronomers – scientist they don’t like to be wrong, but they love to be wrong.
Pamela Gay: Yea, no. We totally blew this one up and it’s looking more and more like we do have gignormous dark matter halos and these are what the large galaxies and superclusters of multiple galaxies all abide within. But smaller systems, the globular clusters, the dwarf galaxies we’re finding don’t have these dark matter halos and I’m just kind of loving this. Because it makes some problems much easier to explain and others harder to explain and anything that gives a cosmologist a challenge that I get to sit back and watch and enjoy being an observer about, yea, I’m good with all of that.
Fraser Cain: Now there are some pretty fascinating kinds of dwarf galaxies that people have identified in the last decade or so and done a lot of observation. So, I don’t know how – I know you’re a dwarf galaxy nerd, but things like the ultra-compact dwarfs, I don’t know if you did any research in – the most dense stars in a galaxy, in the universe, are these tiny little – they’re like globular clusters on steroids.
Pamela Gay: Well, they’re essentially transition objects between globular clusters and galaxies and this is one of those things that leave many of us just sort of like okay, how do we explain all of these things. Because these a lot of relationships that almost make sense with globular clusters being the left-over cores of destructed galaxies which is one of the things we brought up last week, but they’re kinda missing black holes. It looks like maybe, but it’s hard to look and see. Then we find these ultra-compact objects and we don’t know if they have black holes or not and they seem to be another kind of transition object. And without knowing the full history of these objects, without being able to get good images of the early universe to see when did all of these different things come into existence.
We don’t know are these ultra-compact galaxies a transition between large elliptical and dwarf elliptical galaxies. Are they something entirely unique and have their own fascinating history? So, this would fit in with the idea that globular clusters aren’t cores of galaxies and are instead formed in ultra-dense, high-pressure regions. So, dwarf galaxies and globular clusters are really the kinds of things you need to look at side by side. Ursa Minor has the exact same stellar composition as a globular cluster but completely different clematics. How do we get both of these objects? These are the things we can’t figure out, yet.
Fraser Cain: Yea, I mean it’s a great theory to say oh globular clusters are just the left-over cores because they kinda look like the core of a galaxy after everything’s been stripped away, but the part that kinda holds together is the core and so all those globular clusters that we see in the Milky Way, they’re all just left-over remnants – the pits of the fruit –
Pamela Gay: Right.
Fraser Cain: That we couldn’t eat. And one that’s – I think that people think is most likely in the Omega cluster –
Pamela Gay: Omega Cen.
Fraser Cain: Which is pretty much – yea the largest cluster in the Milky Way and they think it has its own supermassive black hole at the heart of it, it really has a lot of those characteristics.
Pamela Gay: It has multiple generations of star formation. And so, the question there is, is it actually a globular cluster or not? So, we look at things like M4, M13, and these classic faint fuzzies you seen in your northern hemisphere telescopes and they have this distinctive look, this distinctive stellar population. Omega Cen, how does it differ from these ultra-compact dwarf galaxies that we see in other systems that we can’t see as close? Is Omega Cen considered a globular cluster only because when it got labeled we didn’t know about the dwarf galaxies all that much yet? And is it like Pluto where it just needs to be renamed as we learn more about this?
Fraser Cain: Yea.
Pamela Gay: This is basically an episode where I’m saying everything we thought we knew we didn’t and isn’t that awesome?
Fraser Cain: Isn’t that awesome. I love it. Okay, so, here’s a question that we’ve been kinda arguing about a bit, which is the large and small magellanic clouds.
Pamela Gay: Yes.
Fraser Cain: Are they dwarf galaxies?
Pamela Gay: They are dwarf irregular galaxies and it’s kind of important to add these extra words. When we talk about a dwarf spheroidal or a dwarf elliptical galaxy, we’re talking very specifically about these little tiny objects that the smallest has single epic star formation. They were theorized to have high dark matter ratios. They’re now theorized totally excited orbitally by our own galaxy. Now the large and small magellanic clouds are bigger, they look more like a dead bug, they have the type of systems you can’t necessarily look straight through. I can show you a picture of a dwarf galaxy and you will be completely oblivious to the galaxy in the frame because they are so diffuse.
Fraser Cain: Right, you can only tell by their movement of the stars all together to know that they are a thing that’s together.
Pamela Gay: Yes, exactly.
Fraser Cain: Yea.
Pamela Gay: And with the large and small magellanic clouds you can just go outside in the southern hemisphere during the right season and they are there looking like someone took a fist to the distant Milky Way and throw it sideways. We see ongoing star formation in irregulars. We see supernova continuing to go off, for instance, 1987, a they’re very, very different systems. So, swarf irregulars, we’re still seeing star formation, we’re still seeing dust and gas. Dwarf ellipticals and spirals have settled out into standardized orbits for their stars. They’ve settled into spiracle or elliptical shapes and probably have very different formation history.
Fraser Cain: And the second question, are they orbiting the Milky Way?
Pamela Gay: No.
Fraser Cain: Maybe. Probably – again, I think if we asked this question, we would have gotten a bunch of answers back and forth. But they are in a gravitational interaction with the Milky Way but still not entirely certain if they’re in orbit around the Milky Way or not. And will they ever collide with the Milky Way?
Pamela Gay: No.
Fraser Cain: Maybe, we don’t know.
Pamela Gay: Well, it all comes down to, again, a matter of mass. We don’t know exactly how much mass our Milky Way has. We don’t know exactly how much mass these systems have and thus we don’t know escape velocities real well.
Fraser Cain: I mean, talk about a new sort of discovery, right. Now, we’ve always said that Andromeda is so much more massive than the Milky Way and now it’s getting closer and closer and closer, so the two galaxies are similar in mass. It’s just that Andromeda has a much more massive – supermassive black hole. So, was there any other sort of newish research in dwarf galaxies that you’ve been finding fascinating?
Pamela Gay: The loss of the belief that they had massive amounts of dark matter is really my favorite thing. There’s a few smaller things, folks are now looking to find the most pristine star, so ones that have the fewest heavy element within them, within dwarf galaxies and we’re using our understanding of the compositions of the dwarf galaxies to figure out how the composition of our own galaxy evolved over time. This being able to trace stars as they transition from dwarf galaxies to being consumed into our own Milky Way helps us understand the disparity between the composition of stars in different points of our own galaxies disc. We’re starting to build a picture and it’s a cool picture of how galaxies build up and change over time. We’re just missing the first few chapters.
Fraser Cain: One of the things that’s really fascinating that’s coming up shortly is the Guya mission has been gathering tons and tons of astrometric data about the motions of the stars through the galaxy. And it is the machine for finding these dwarf galaxies because as we said you’re looking for stars that are all moving together in the same direction either within the galaxy or even outside the galaxy. There’s a wonderful picture that came from Guya, that is a recreation of the Milky Way and the largest small magellanic clouds, but it’s not a really a photograph. It looks like a photograph but it’s actually data from the spacecraft. And so, this is just the most phenomenal machine for finding these kinds of objects, so I wouldn’t be surprised when the final Guya data release comes out in a couple of years that they’re just going to turn up so many new dwarf galaxies, both the remnants – the title tail remnants inside the Milky Way as well as the ones that are out there orbiting around us.
Pamela Gay: Guya is one of the very few spacecraft I let myself get super excited about before it launched because it is just such an amazing piece of technology. And gods speed Guya, keep doing your job.
Fraser Cain: Haven’t we done an episode on Guya? I’ve done an episode on Guya especially about the new data release.
Pamela Gay: It came up in one of our planet finding episodes.
Fraser Cain: Yea, but I – it’s definitely worth us now doing a deep dive into Guya because there’s so many great results coming out of it. It – I wouldn’t be surprised if it ends up being one of the most productive space missions ever launched. It’s amazing.
Pamela Gay: Things to work on for 499 when we come back from hiatus.
Fraser Cain: There you go, next season. Alright, well, Pamela, as always, it’s been an honor and a pleasure to do Astronomy Cast with you. I hope you have a great summer and I can’t wait to see you and all of the fans in Edwardsville, Saint Louis, in September.
Pamela Gay: September 15th and 16th and I can’t wait for you to be here and to raise a glass in my favorite pub when there isn’t a tornado going on.
Fraser Cain: Which is like never. Alright. Alright, we’ll see you in September.
Pamela Gay: Buh-bye.
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Duration: 30 minutes