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It’s time to begin a new mini-series, where we’ll look at different classes of galaxies. Today, we’ll start with the dwarf galaxies, which flock around larger galaxies like the Milky Way. Are they the building blocks for modern structures?
Transcript
Human transcription provided by GMR Transcription
Fraser Cain:
Astronomy Cast, Episode 718: The Galaxy Series – Dwarfs. Welcome to Astronomy Cast, our 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 the Planetary Science Institute and the director of CosmoQuest. Hey, Pamela. How are you doing?
Dr. Pamela Gay:
I am doing okay enough. I am recovering from discovering shampoo I am terribly allergic to. So, there was chaos on our normal recording today. And I continue to be in a bit of a Benadryl haze but I’m better than I was yesterday. So, thank you everyone for your patience.
Fraser Cain:
All right, well hopefully you’ll be here cognizant and ready to explain galaxies. Now I did one of my live streams last night, and I was talking about different kinds of telescopes, blah, blah, blah. And I always recommend the first thing you do is you get a pair of binoculars. And there’s a sale on for Celestron Sky Masters. So, the 20x80s are down from 200 US to 140 US on Amazon.
Dr. Pamela Gay:
Oh wow, that’s 50%.
Fraser Cain:
Yeah, almost. And then similar for the 25×70. And so, I’m not sure what the story is. There’s no ad. We’re not sponsored. I just noticed this. And so, if anyone is like “Oh, I really want to pick up a pair of astronomical binoculars” I don’t know if the sale will last but they seem to be relatively inexpensive on Amazon in the US. That is the beginning and the end of this message.
It’s time to begin a new mini-series where we’ll look at different classes of galaxies. And today we’re going to start with the dwarf galaxies which flock around the larger galaxies like the Milky Way. Are they the building blocks for the modern structures that we see all around us? So, give me an example of a dwarf galaxy that maybe people in the southern hemisphere are familiar with. Give me two.
Dr. Pamela Gay:
In the southern hemisphere they have the large and small Magellanic Clouds. These are dwarf galaxies that look like someone grabbed a handful of the plane of the Milky Way and just tossed it to the side. These are two systems where it’s not entirely understood and there is debate in the literature over whether or not they are going to end up in orbit around our Milky Way or whether or not they’re going to end up flying past. And I feel like every few years they change their mind.
Fraser Cain:
Right, it’s like the question about whether or not the sun is going to consume the Earth. We get that going back and forth as well. So, give us a comparison. How big and massive is a dwarf galaxy compared to something like the Milky Way?
Dr. Pamela Gay:
They can get so tiny. So, we have systems like the Ursa Minor dwarf spheroidal galaxy which is one of the smaller ones that are the size of globular clusters. They have masses of hundreds of thousands of stars. The majority of that is actually dark matter. And then they get up to being fractions.
Fraser Cain:
Like 2%.
Dr. Pamela Gay:
Like 10% of the size of a spiral galaxy like us. Now to be clear, the large Magellanic Cloud is a dwarf spiral barred according to the latest classifications. So, I feel like there’s folks that are going to be @-ing us saying “But I have heard”. It’s still a dwarf folks. It’s still a dwarf.
Fraser Cain:
It’s a jumbo shrimp.
Dr. Pamela Gay:
Yes, exactly.
Fraser Cain:
Right. So, okay, and then apart from the LMC structurally what do these dwarf galaxies tend to look like in terms of regular matter stars, gas, dust, and dark matter?
Dr. Pamela Gay:
So, there are basically the smallest ones that are dark matter dominated and have globular cluster-ish masses, and then there are the larger ones which were able to undergo multiple generations of star formation and have normal to low amounts of dark matter. So, just like low luminosity galaxies can have squirrely amounts of dark matter, dwarf galaxies can have squirrely amounts of dark matter. And there’s lots of debate about how this ends up being the case.
The story that seems to be coming together in the literature is that on the small side of these, you have that first generation of supernovae that goes off and it is able to blast large amounts of the baryonic matter out of the halo of these dwarf galaxies. When you start looking at them in molecular lines you see lots of cold gas outside the system’s core, and they’re dark-matter-dominated. So, this seems to communicate that through supernovae and other actions, they blast out most of their luminous matter, most of their baryonic matter. Some of that gets blasted out at escape velocities. And what’s left behind is essentially a halo of dark matter hanging out darkly in the halo of the galaxy.
Fraser Cain:
So, it’s kind of like a star or a stellar nebula when it starts to finally form, and the stars start to turn on, and then their stellar winds blow. And they clear out all that gas and dust. There’s these three-body interactions with stars whipping around each other but in a large galaxy like the Milky Way, because the gravity is so intense the escape velocity is very high. And so, these stars are stuck. They’re just not near the cluster. They escape the cluster, but they don’t escape the galaxy. But I guess in these dwarf galaxies there’s so little gravity holding the thing together that it can shed bits and pieces of itself out into space quite easily.
Dr. Pamela Gay:
And this gets people asking questions along the lines of “What is the difference between a small dwarf galaxy and a globular cluster?” And it appears to be one strictly of how they formed. We’re now starting to understand that globular clusters most likely form during galaxy interactions where material gets slammed together. And where these shockwaves come together you get globular clusters forming. Whereas dwarf galaxies form kind of like an open cluster but large.
You have this massive cloud of material that collapses under its own gravity. And as it does this it’s able to start having star formation. And so, you have a dark matter halo, luminous material gravitationally pulled into the center, star formation, and supernovae can blast material out if it’s too small. Multiple generations of star formation can go on if it’s large enough. And yeah, they’re just cool little systems.
Fraser Cain:
With the large Magellanic Cloud though, the largest regions of star formation that we know of in our near vicinity are in that galaxy or in that dwarf galaxy. Think about the Tarantula Nebula. It’s a ludicrous amount of star formation, stars vastly more massive than anything we know of in the Milky Way. Is that just a special case?
Dr. Pamela Gay:
It’s big.
Fraser Cain:
It’s big, yeah.
Dr. Pamela Gay:
This is where we start getting into the jumbo shrimp category.
Fraser Cain:
And I guess the tidal interactions too with the Milky Way.
Dr. Pamela Gay:
Well, so you have a number of different things going on. This is a system that has its own globular clusters that is indeed interacting with the galaxy, our galaxy. There are occasional questions of were the large and small Magellanic Cloud once the same thing. I don’t think that’s ever come to a consensus. But the question does get asked which amuses me. And with all of these different interactions going on, interactions trigger shockwaves, trigger star formation. And so, as a system sweeps past the Milky Way, as it interacts with our dark matter halo, as it starts to interact with our outermost actual stuffs, the baryonic stuffs, you’re getting shocks that trigger star formation. And it’s glorious to look at.
Fraser Cain:
Yeah, yeah. So, then let’s talk about where these things came from. Do we know where or how dwarf galaxies originated? Because I’m sort of thinking about say a star cluster where you’ve got this giant stellar nebula, like the Orion Nebula, and you’ve got concentrations of larger amounts with larger stars forming and then you’ve got smaller areas. Maybe you’re even getting these rouge planets forming. And so, when we think about the primordial hydrogen and helium in the early universe, is it the same idea as a stellar nebula writ large with large blobbers turning into certain-sized galaxies and smaller blobbers turning into the galaxy equivalent of red dwarf stars?
Dr. Pamela Gay:
So, I’m just going to clean up the language here a little bit.
Fraser Cain:
You don’t like blobber?
Dr. Pamela Gay:
No, that’s fine. I’m down with blobber.
Fraser Cain:
All right, all right, all right.
Dr. Pamela Gay:
So, solar nebulae generally form to solar systems forming. We have star-forming regions which are a kind of nebula. I’m not sure what you’re referring to with the stellar nebula.
Fraser Cain:
Yeah, a star-forming region like the Orion Nebula where you’ve got this giant area of gas and dust of different concentrations and different stars whirling up inside this whole region of different masses. There is this mass relationship in a star-forming region that creates stars of different sizes. So, is it the same mechanism to make bigger galaxies versus dwarf galaxies?
Dr. Pamela Gay:
No, because star-forming regions are singular clouds that have been shocked into simultaneously forming all of these stars. And you can have dwarf galaxies like Ursa Minor where all of the gas in them got shocked into a single star-forming burst and whatever wasn’t used up got blasted away. But they didn’t form that way. We think that the modern dwarf irregulars that are rich in star formation are actually analogs to these dwarf blue galaxies that we’re starting to see in JWST images.
Where we’re seeing these smallest of the dark matter halos that formed are pulling in material and that material just like in any form and galaxy has the chance to then clump up. And some of those clumps are going to form stars now, some will form later. And in the case of the dwarfs, there’s just not that much material. So, you don’t tend to get the same numbers of star-forming epics that you get with bigger systems. But it’s that same dark matter halo collects material inside. Material forms galaxies. And these are essentially building blocks of large galaxies that came later.
Fraser Cain:
Right, right. But I guess let me try and kind of rephrase the question because I’m imagining, in the beginning, there is just the primordial hydrogen and helium. There is the dark matter structure that underlies the whole thing. And you’ve got different concentrations based on regions of over and under density in the original universe. And then gravity is pulling things together. And so, places where you’ve got an over density, you’ve got more stuff being pulled together, and in places you’ve got an under density you’ve got less stuff being pulled together.
And after a while like pizza dough that’s being pulled too thin, you start to get gaps opening up, voids opening up. And then as you pull farther and farther then these things kind of snap and the gravity pulls them together into however much gas you have. And then the expansion continues. And so, now you’re left with this just distribution of blob – I’m going to go back to my blobbers – of material that is now gravitationally distinct from the other ones.
They’re still orbiting one another but they are not continuing to merge up into larger objects in the beginning. Is that roughly on the right track then? Are the dwarf galaxies that we see today the result of those sheared-off chunks of primordial material or was there some mechanism later on that spun them out that split them in half into smaller pieces?
Dr. Pamela Gay:
So, things don’t get split up later generally unless it’s like a tidal tail or something and those probably aren’t forming dwarf galaxies. So, the way to think about it is things tend to break up into pieces in a distribution. If you drop a mug, you’re going to get a distribution of pieces where you’ll have a few giant pieces and then inevitably a whole lot of little, tiny crumbs which is always the source of sadness when trying to reassemble the mugs.
Fraser Cain:
Powder.
Dr. Pamela Gay:
Yeah. And when the early universe fragmented we had a distribution of blobs of material, going back to your blobs. And there were a few that were giant. And these were those first forming giant galaxies. But the majority were these little, tiny lumps of extra material that pulled in stuff. And then those within them could fragment further. So, you have all this stuff is gravitationally bound together.
But within these gravitationally bound together, you could get further fragmenting into star-forming regions that would start up stars at different points, different times, due to what triggers were there to start the star formation. So, the entire universe has slight over and under densities. It fragments into pieces. Big pieces form giant galaxies. The majority is little pieces forming little galaxies that are gravitationally bound. And within that gravitationally bound you could get further claps into star-forming regions.
Fraser Cain:
Thanks to Gaia, the amazing Gaia mission we can see the evidence of the dwarf galaxies that the Milky Way has consumed in the ancient past.
Dr. Pamela Gay:
And now we’re a very hungry galaxy.
Fraser Cain:
Sure, but it is interesting that a lot of the larger mergers – like I think the last great happened eight billion years ago. So, you say it’s happening now, but it sounds like it was furious in the early universe and now it’s a lot less common as the universe matures into an old age. All right, one of the big questions that Webb was designed to ask was will we see these dwarf galaxies coming together as building blocks to the larger galaxies? Those observations are now happening. What is this story that we’re seeing of galactic evolution over the entire age of the universe?
Dr. Pamela Gay:
We’re still figuring out all the details. What we do see is there are these small bright blue furious with star-formation galaxies in the early universe. JWST is finding them. What we see is there are galaxies that are undergoing greater amounts of merger activity in the past. There are more quasars in the past. And that excess material to feed black holes, that significantly more galaxy mergers going on the past, what is happening is over time gravity is pulling things into tighter and tighter structures. So, we went from lots and lots of small stuff to the small stuff having time to consume one another, to building bigger things, then kept eating the smaller things.
And so, we have this picture where some galaxies did just form giant from day zero. But the majority of systems grew through the constant merger of larger and larger systems. And the shapes of the galaxy seem to be dominated by what were the angles that they came together. What were basically the distribution of big things to small things orbiting it for grand spirals? So, a lot of these grand design spiral galaxies that we see are driven by having a smaller companion. So, here we can thank dwarf galaxies in many cases for bringing us grand design spiral galaxies.
Through this constant merger of systems, we get bigger and bigger things. And this is an ongoing process. We continue to eat dwarf galaxies in lower numbers because we’ve already ate so many of them. There’s just not as many left to be eaten. We continue to eat them. And then we’re also seeing the larger systems merging together. And this is where things get more and more interesting as time progresses because we’re going to run out of things to merge eventually. And so, it’s small galaxies merging bigger and bigger all the way down.
Fraser Cain:
I mean I think there’s sort of two pathways that does really interesting things to JWST. And the first one is this idea they’re called the impossible galaxies. They’re not impossible. They’re possible. But the gist is that we’ve got these big galaxies early on in the universe. You’re seeing quasars at less than a billion years after the Big Bang. You’re seeing spiral galaxies again literally within the first billion years of the universe. And so, that is definitely pushing things back to what you mentioned, that you get large chunks are just turning directly into big galaxies.
And then also this history thanks to Gaia of seeing when the mergers happen. That a lot of the big mergers happened early on in the Milky Way’s history. And that is just driven by these dwarf galaxies. And so, what do you know, it’s more complicated than we thought. That it’s probably both. That you’re getting the dwarf galaxies being the building blocks of the bigger galaxies. And a lot of them are just never making it close to another galaxy and are remaining unperturbed since almost the beginning of the universe.
Dr. Pamela Gay:
And the difficult thing is they are hard to see. And when these smaller things are the most numerous and the smallest things tend to be the most dark matter-dominated it becomes very difficult to do a census of these beyond our own local group. We know dwarf galaxies dominate. We know that galaxies, large ones participate in dwarf tossing using their gravity on a regular basis.
And so, this is the hardest to observe kind of galaxy and also the most common and also the most necessary to understand. What I love is as we try and understand our past, different things that we’ve known for a long time like the Oosterhoff classifications of galaxy clusters where half of them – not half but a lot of them but a lot of them are going in entirely the wrong direction.
And we have this thick disc to our galaxy that has a slightly different metal composition. That is all getting tracked back to eating larger dwarf galaxies and consuming their globular clusters, consuming their matter. We are a system made from multiple galaxies coming together and sharing their material and their angular momentum and everything else to give us this barred spiral structure. And that bar is due to our companions.
Fraser Cain:
Right, yeah. Now you talked about sort of how some of these galaxies are dark matter dominated. In other cases, it’s the opposite. They have very little dark matter. So, what is the mechanism that is creating such vast differences in composition for these different dwarf galaxies?
Dr. Pamela Gay:
It likely comes down to different interactions. This is what we’re also finding with the low surface luminosity galaxies where if you have a system or two systems that pass through each other you can end up with the dark matter staying in one part and some of the luminous matter escaping. And so, you get systems that are luminous matter dominated, you get systems that are dark matter dominated, and of course, the holy grail is finding the system that is just dark matter. We haven’t quite got there yet. We’re getting close with some of these systems. The luminous to dark matter ratio is in the hundreds with some of these dwarf galaxies.
And this is where it starts to get necessary to look in every wavelength to find all right is there just super cold gas in these systems that is giving them mass that we don’t otherwise see? Because cold mass is transparent and not exactly luminous and optical. So, millimeter dishes are so important for studying these and getting down, looking for the molecules. And they’re cool and hard to see and don’t get the attention they deserve because they’re not necessarily the kind of thing you write press releases with pretty pictures about.
Fraser Cain:
Yeah. It is amazing to me how often new dwarf galaxies are being discovered. That every year or so I feel like there’s a couple of new dwarf galaxies that turn up from larger and larger surveys of our neighborhood. And some of these ones as you said that are dark matter dominated are harder to find. And so, often it’s like gravitational lensing gives us insights into where these things are. And I know you mentioned early on there’s one that as you said is the mass of a star cluster.
Dr. Pamela Gay:
Yeah, there’s several like that. Yeah.
Fraser Cain:
And yet is a galaxy with all the parts and pieces.
Dr. Pamela Gay:
The Ursa Minor dwarf spheroidal galaxy is near and dear to me. It was the topic of my master’s thesis. If you take an image of it – it’s about a degree across so you need a big field of view – you can’t tell you’re seeing a galaxy. It’s just an overdone stay of stars in the sky. And it’s closer to us than some of our globular clusters. So, these are large diffused systems where you can see individual stars and look right through them to galaxies beyond. They’re super cool.
Fraser Cain:
Since when you did your doctoral thesis, there have been many more even lighter, even smaller galaxies. It’s kind of amazing. So, for example, there’s one called Segue 2 that has about 1,000 stars. That’s it, 1,000 stars in a galaxy. So, again this is the scale of what’s out there. And yet the galaxy itself has about 55,000 times the mass of the sun.
Dr. Pamela Gay:
Dark matter dominated.
Fraser Cain:
Dark matter dominated, yeah exactly. That is causing the bulk of the mass in that galaxy. It’s fascinating. And I wish they were easier to see because they are telling the true story of the history of the universe. And this is one of the things Webb was really designed to do was to find these things and see them coming together to really tell us this story of where we came from. And there couldn’t be a more cutting-edge piece of research right now. That when we look back and we think “Oh what are the things that changed dramatically?” I’ll bet you the story of dwarf galaxies will be one of the ones that we’re going to come back to in 10 years and go “Oh we didn’t know anything about dwarf galaxies.”
And thanks to all of these Euclid and Vera Rubin and Nancy Grace Roman and the DESI Database, there are all these things that are coming online to search for dark matter or dark energy and do these detailed galaxy surveys of the area around us. And I think our understanding of the universe is going to change subtly or dramatically in the coming years. And so, don’t be surprised if we go like “Dwarf galaxies, we hardly knew you.”
Dr. Pamela Gay:
And what’s amazing is what is nearly invisible today because they are made of elder red stars. They shone like fireflies in the early universe. These were rich in blue stars billions of years ago. And so, how our universe looks has radically changed with the single epic of star formation dying out in all these systems. So, they were the bright blue galaxies of the past. And they were the red almost impossible-to-see galaxies of today.
Fraser Cain:
All right, so stay tuned to this series. We will be back next week with the next episode. Thanks, Pamela.
Dr. Pamela Gay:
Thank you, Fraser. And thank you to everyone out there who is donating at the $10 a week or higher level. Thank you actually to everyone that donates. I just want to be clear; we are grateful for all of you, but I only mispronounce the names of folks donating at the $10 or higher level.
Fraser Cain:
What a special reward.
Dr. Pamela Gay:
This week I’m going to desperately attempt and fail to pronounce the following people-who-I’m-grateful-for’s names. Thank you to Paul L Hayden, Steven Coffey, Bart Flaherty, Benjamin Carryer, MHW 1961 Super-Symmetrical, Micheal Purcell, Jim Schooler, Schercm, Andrew Stephenson, Tim McMackin, The Lonely Sand Person, Kenneth Ryan, Gregory Singleton, Frode Tennebo, Micheal Regan, Father Prax, J Alex Anderson, Glenn McDavid, Jim McGihon, Bruce Amazeen, Cemanski, Planetar.
The Air Major, Marco Iarossi, Matthew Horstman, Scott Kohn, Scott Bieber, Georgi Ivanov, Justin Proctor, Matthias Heyden, Lew Zealand, Naila, The Big Squish Squash, David Gates, Benjamin Muller, Cooper, Eran Segev, Peter, Philip Grand, Don Mundis, James Rodger. Wow, that one I shouldn’t have to stumble over but I’m going to today.
Fraser Cain:
It’s a perk.
Dr. Pamela Gay:
Yeah, it is. Sean Martz, Camy Raissian, Nate Detwiler, Sam Brooks and his mom, and Dean. Thank you all so very much. You make this show possible.
Fraser Cain:
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 attributions 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. This episode was brought to you thanks to our generous patrons on Patreon. If you want to help keep this show going please consider joining our community at patreon.com/astronomycast.
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