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 th