All About Dwarf Galaxies: A Conversation with Astronomer Charlotte Christensen

Transcript

Marshall Poe:

Welcome to the New Books Network one. Hello everybody. This is Marshall Poe. I'm the editor of the New Books Network, and you're listening to an episode in Grinnell College's Artists and Authors podcast. And today I'm very pleased to say that we have Charlotte Christensen on the show, and she is a professor of physics at Grinnell College. And I'm very interested to talk to her because I read a lot of physics books. I'm not sure how much I understand of them, but I'm hoping that she'll be able to clarify some things for you and for me. Charlotte, welcome to the show.

Charlotte Christensen:

Thank you so much for having me.

Marshall Poe:

It's my pleasure. Could you begin by telling us a little bit about yourself?

Charlotte Christensen:

Sure. Well, as you said, I am a professor, associate professor at Grinnell College in the physics department. So I'm technically a physicist, but I actually consider myself more of an astronomer. My PhD is in astronomy from University of Washington, and I arrived at Grinnell College about eight years ago after a postdoc at the University of Arizona. And now I am here at Grinnell. I'm just getting off my sabbatical, following tenure, which has been a true delight.

Marshall Poe:

Oh, congratulations.

Charlotte Christensen:

Thank you. Yeah. And I'm looking forward to getting back in the classroom in the fall.

Marshall Poe:

That's great. I'm sure the people at Grinnell will be happy to have you back. Could you tell us a little bit about your research focus, what exactly you do?

Charlotte Christensen:

Sure. So, broadly, I'm interested in how galaxies forum and how galaxies evolve. So when we look at the galaxies and the universe, we see that there's this huge diversity of galaxies. Some of them are huge, like these giant elliptical galaxies, which can be maybe a thousand times the mass of the Milky Way. And some of them are really tiny. These are known as dwarf galaxies. And you might be familiar with, say, the Large Magellanic Cloud or the Small Magellanic Cloud, which are two dwarf galaxies.

Marshall Poe:

I wish I could say I was familiar with that, but I'm not.

Charlotte Christensen:

Well, if you lived in the Southern Hemisphere, you would be because you can actually see them there. But there're two small galaxies that orbit the Milky Way just the way that the Earth orbits the sun. And they have around 1% to a 10th of 1% the mass of the Milky Way. So what I'm specifically interested in are these really teeny galaxies, and I want to know why they act the way they do. One thing we notice is that the small galaxies that orbit the Milky Way look really different than small galaxies which are, say, isolated, far from any more massive galaxy. So we'd call those isolated galaxies or field galaxies in comparison to the satellite galaxies. And we see the galaxies that are isolated tend to still be forming stars, but satellite galaxies, the Milky Way usually aren't. And one of the questions I'm interested in at the moment is why this happens and what exactly shuts off the star formation.

So I did all this work using computers. I am not an observer. I never go out and look through a telescope for my own research. I read other people's research that uses telescope data, and I try and incorporate that into my own work. But what I do is I simulate how galaxies form. So I simulate a box of the universe, I include gravity and things about the gas, such as the density and the temperature and things about how star formation happens and what happens when stars die in supernovae. And all of that physics goes into the simulation. And I basically see what happens and try and use that to help us interpret what's actually happening in the universe.

Marshall Poe:

So let's take a step back, deep back time and work toward the simulations. So the universe, if I'm not incorrect, it's about 14 billion years old.

Charlotte Christensen:

That's correct.

Marshall Poe:

Is that right?

Charlotte Christensen:

Yeah.

Marshall Poe:

Something like that, yeah. And at that point, this thing, the Big Bang happened, and then there was something called expansion.

Charlotte Christensen:

Yes.

Marshall Poe:

Is that what it's called?

Charlotte Christensen:

Yeah. We talk about the expansion of the universe.

Marshall Poe:

So the one thing I don't really understand, and this is a kind of digression that I've always wanted to ask an astronomer, is the expansion happened very quickly. Is that right? I mean, it was like there was nothing and then there was something. And did things move faster than the speed of light at that time? I mean, how did that happen?

Charlotte Christensen:

So I think what you-

Marshall Poe:

Are these naive dumb questions? They may be, I don't know.

Charlotte Christensen:

No, these are great questions. I think what you're referring to is the inflationary period.

Marshall Poe:

Inflationary period. That's it.

Charlotte Christensen:

Yeah.

Marshall Poe:

That's exactly right. See, I had the terminology wrong. Yes, the inflationary period.

Charlotte Christensen:

Right. So when we think about the history of the universe, the whole universe is getting bigger, but it's been getting bigger at different rates depending on when this is.

Marshall Poe:

Yeah, here we go. Okay.

Charlotte Christensen:

So right after the Big Bang, the universe expanded at an exponential rate very, very rapidly during this period called inflation. And then it sort of stabilized for a while and kind of coasted along, still expanding, but at a significantly slower rate. And now what's happening is that the rate of expansion of the universe is actually increasing again. It's still not anywhere near as rapid as it was during the inflationary period, but this still, this is a mystery. Physicists can't explain why the universe would start to speed up, and this is why physicists refer to dark energy. Dark energy is whatever it is that is causing this accelerating expansion. So there are all these different periods.

Marshall Poe:

We don't know why the rate of expansion is increasing. Do we know why the inflationary period slowed down?

Charlotte Christensen:

Well, there are theories there.

Marshall Poe:

I bet there are.

Charlotte Christensen:

And it's honestly not my field. So I think I'm not going to-

Marshall Poe:

Defer.

Charlotte Christensen:

... go through all of those. Yeah. I'll defer to people who study that more closely.

Marshall Poe:

That's a very good answer. So at some point, gravity becomes involved. And is this how galaxies are formed?

Charlotte Christensen:

Yeah. Yeah. The very early universe had pockets that were higher density and pockets that were lower density. I actually think, this getting back to inflation, that those pockets of higher and lower density are because of actually quantum fluctuations in the initial universe that then got expanded out to huge scales during inflation. But regardless, if we think about the time just before galaxies started to form, when the universe was starting to cool down, the areas that were higher density would collect more mass into them because of gravity, and the areas that were lower density would lose mass. This is the rich get richer and the poor get poorer. And over time, the areas where there is higher density continue to collect more and more mass as gas cools down, and as gas cools into what we call these regions of higher density, mostly driven by dark matter, actually, that gas can form stars. And that's how the first galaxies formed.

Marshall Poe:

And these are largely gravitational processes by which these areas of high density create kind of gravity wells and then things flow into them in an ever increasing rate.

Charlotte Christensen:

That's exactly right.

Marshall Poe:

Is that right? And stars come out of this process as well, is that right?

Charlotte Christensen:

Right. Yeah. So stars happen when the gas reaches sufficiently high density, and that only happens because gravity is driving all that gas in together.

Marshall Poe:

And then once you have stars, then you get the production of what we call elements. Is that right?

Charlotte Christensen:

Sure. So the initial universe was primarily, in terms of its regular matter, what we would call baryonic matter, was hydrogen and helium with really trace elements of lithium. But obviously you and I, and basically everything we interact with has lots of other elements in them like, for instance, carbon and oxygen. And those elements are produced by stars either during the star's lifetime or when a star dies in a supernova.

Marshall Poe:

So where do we get... I understand the inflationary period, and I understand the expansion and then slowing down and then re-expansion, which is going on now and is inexplicable. How do we get galaxies, which are... what are galaxies? How about that? What's a galaxy?

Charlotte Christensen:

So like many things in astronomy, the definitions get more complex the more you think about it. But I would say, at the most basic level, a galaxy is something that a region of dark matter, which is all gravitationally bound together along with some stars in there. Dark matter, to back up a little bit, dark matter is matter that light doesn't interact with. So that means that this matter doesn't absorb light and it also doesn't reflect light. And for that reason, astronomers can't see it using, say, photons detected through a telescope. But we can see it's gravitational influence on other matter. And we see that the galaxies primarily consist of dark matter. Dark matter might be something like, oh, 90% the mass of a typical galaxy. And then the rest of that galaxy is stars and gas. Not all dark matter, halos or collections of dark matter actually contain stars. So those are things that we don't consider galaxies, but they do exist out there.

Marshall Poe:

And these are just big gravitational wells. And we know they're there because...

Charlotte Christensen:

I would say they're small gravitational wells.

Marshall Poe:

Okay. All right.

Charlotte Christensen:

So on the order, well, it's all relative, so think about, say, a 100,000 times the mass of the sun.

Marshall Poe:

Yeas. A small... Wow.

Charlotte Christensen:

And we theorize they're there. We haven't actually detected these yet.

Marshall Poe:

Right. So once you have a galaxy, I mean, we're pretty familiar with pictures of galaxies. I think they're especially called spiral galaxies. I think the Milky Way is one, right? It looks like a spiral.

Charlotte Christensen:

That's right.

Marshall Poe:

But not all galaxies are spiral galaxies. Is that right?

Charlotte Christensen:

That's right. Yeah. So the Milky-

Marshall Poe:

Why are some spiraling and some aren't? Do we know that?

Charlotte Christensen:

So this gets at the heart of my research.

Marshall Poe:

Here we are.

Charlotte Christensen:

Yeah. So many galaxies are spiral galaxies, and these are galaxies which are basically disc shaped, or the gas and the stars is in a disc kind of like a Frisbee, and it's all rotating around together and the rotation leads to the creation of these spiral arms. But other galaxies, say, more massive galaxies might be more egg shaped or elliptical, and actually less massive galaxies as well. And there's a big question about how galaxies transition between these different types. One thing that's probably very important to this is collisions between galaxies. Collisions between two spiral galaxies is one good way to form an elliptical galaxy. And repeated collisions are probably even better. At they're really low mass end where I'm interested in, it might be that some of these galaxies never form a disc at all, they're just always sort of spherical or elliptical shape, or there might be something else happening with the dynamics.

Marshall Poe:

Why do spiral galaxies spin at all?

Charlotte Christensen:

So this is-

Marshall Poe:

What's up with that?

Charlotte Christensen:

So if I say the words conservation of angular momentum...

Marshall Poe:

Yep. I get that.

Charlotte Christensen:

Hopefully that won't scare anybody off and might even bring back recollections of, say, an intro physics course. So this idea is that any spin that an object has is conserved, so long as there's no external force stopping that spin. And the rate at which something spins increases as that object gets smaller. So this is the classic ice skater who starts [inaudible 00:13:03]

Marshall Poe:

The ice skater. I was thinking about the ice skater. Yes.

Charlotte Christensen:

Yep. Raise the arms in and the spin increases. Galaxies work the same way. We think in the early universe, all that gas out there just because of random fluctuations had a little bit of initial spin to it. It was very, very slow. But as that gas collapsed down into these gravitational wells, that's been increased and now is what causes the galaxies that formed out of it to rotate.

Marshall Poe:

So this is a bit of an aside, but the dwarf galaxies that you study, do they spin?

Charlotte Christensen:

They do, yes. They spin as well, but not necessarily as uniformly as a disc galaxy. Their stars, rather than all rotating together in a single plane, might look more like a cloud of bees or something, all orbiting together.

Marshall Poe:

Yeah. I got it. And you mentioned that galaxy collide. If one galaxy is bigger than another and they're in the reasonable proximity, then we can predict that the big one will eat the small one. Is that right? And is that going on?

Charlotte Christensen:

Yeah. So we talk about galactic cannibalism. Actually we use those wording, where the galaxy eats a smaller galaxy. How these collisions look and what the end result is, it depends a lot on the initial state of these galaxies, how they are moving in relationship to each other, what their sizes are in relationship to each other. Sometimes galaxies of fairly equal mass collide. We call this a major merger. And we think this is what's going to happen between the Milky Way and the Andromeda galaxy.

Marshall Poe:

Oh, great. We have something to look forward to. Well, here's a question for you, and again this will betray my lack of knowledge about physics. When I take two things and throw them together, you do end up with one thing, but a lot of stuff gets spit out as well. Does this happen with galaxy? It's like somebody just gets ejected as a result of the force of bringing them together.

Charlotte Christensen:

We think so, that some stuff could get ejected. The vast majority of material is going to end up in this final conglomerate galaxy, but there could be stars that end up outside the galaxy.

Marshall Poe:

Right. A poor lonely star wandering without a galaxy.

Charlotte Christensen:

Very much so.

Marshall Poe:

Well, it will find another galaxy to eat it eventually, right?

Charlotte Christensen:

Probably not.

Marshall Poe:

Oh wow.

Charlotte Christensen:

And this is getting back to the expansion of the universe. Because the universe is getting bigger, galaxies in general, though not in all cases, are getting farther apart.

Marshall Poe:

I have to ask this question, and I confess this perplexes me. Gravity is a very strong force. And so, is it the case that, well, I don't even know how to say it, it would seem that the universe would collapse back on itself at some moment, wouldn't it?

Charlotte Christensen:

Yes. This is one of the big mysteries at the heart of cosmology is we would think that a universe filled with matter, all that matter would bring it back together, would cause it to collapse. And that's just why we have to hypothesize dark energy. We need something else which causes it to keep on moving apart and keep on moving apart at a faster speed.

Marshall Poe:

So dark energy, it's necessary for the theory. And a critic might say it's a fudge. Do you have any opinion on that?

Charlotte Christensen:

I think I would say that we've discovered a mystery, and-

Marshall Poe:

Yeah, we definitely have.

Charlotte Christensen:

We know it has to exist. It's not a fudge to make our results work out so much as a sign that we need to learn something new.

Marshall Poe:

Yeah, right? I mean, it's inferred by all the other more or less demonstrated facts about the universe. So it's a good inference that it's there. So I want to talk a little bit about the way that you do research, because as you said, this is fascinating to me, you don't actually do observation yourself, you rely on other people's observations and you use computer models. And one of the things I was reading about your research is that... I'm a historian, and one of the problems with history is you can't go back and run tests. It happened, and that's it. You can't go rerun the French Revolution, adjusting variables to see if it turns out differently. So you have to do something else. And it seems to me there's kind of an analogy between that example and what you do. Would that be correct?

Charlotte Christensen:

Yeah, that's very much correct. So we can't rerun our universe, and we also can't go back and, say, examine a galaxy directly from the past, the way you can't go back and talk with a soldier from the French Revolution. What we can do in astronomy is we can see light from more distant galaxies, which was produced by those galaxies in the past. But we can't watch a single galaxy of evolve over time, even. So what simulations let us do is they let us play with toy universes where we can change the physics and we can examine a single galaxy over its entire history, and we can get really into the nitty gritty of that single galaxy, information that one would not be able to easily get from observations. The only problem is we have to prove that our simulations are realistic in order to make [inaudible 00:18:49] from them.

Marshall Poe:

Yeah. So the reason you can't watch a galaxy evolve is because the times spans are too long. Is that correct?

Charlotte Christensen:

That's right. Galaxies evolve over billions of years.

Marshall Poe:

Right. That's a problem. And then in the construction of, because I'm trying to think about, as a historian I could create a simulation of the French Revolution and go adjust variables, I'm sure someone is doing this in some way, some historical sociologists has done this already and they change various things, and then you get what we call in history counterfactuals. This didn't happen, but could have. And you do this by adjusting variables. What variables are involved in your research? What are the inputs that you tinker with in order to determine whether you have the proper model of a galaxy?

Charlotte Christensen:

So, some of the things that are very important in the models have to do with descriptions of how stars form and also the descriptions of how energy from supernova is transmitted back to the galaxy. Some of my previous work had to do with looking at models of the gas that stars are formed out of, specifically in molecular hydrogen, and trying to better describe that. My current research, I don't so much change the physical models. We have a set of physical models that we think are pretty good, but instead I look at galaxies that are formed in different environments. So I compare similar galaxies that one might be close to something like the Milky Way, and one might be very far from it. And I see how those differ.

Marshall Poe:

Well, there's also another analogy from my discipline here is that the French Revolution wasn't the only revolution. You can compare it to other revolutions. And people have done this. So this is essentially, I don't know, essentially, that might be the wrong word, so this is what you do, you take different galaxies in different places in different environments, and you compare them. Is that right?

Charlotte Christensen:

That's exactly right.

Marshall Poe:

Does this then work toward a kind of general theory of galaxy formation, or is it specifically about dwarf galaxies?

Charlotte Christensen:

My work is specifically on these low mass galaxies, but my hope is that the information we get from them will be applicable, with some modification, to larger mass galaxies too. It's just low mass galaxies seem to have very specific formation processes. And part of that is because they are so low mass, they're really sensitive to anything happening around them. Their gravitational hold is not very strong. So something like the presence of a more massive galaxy or a close encounter with another low mass galaxy can really, really change them.

Marshall Poe:

How big is the data set here in terms of dwarf? I'm trying to think, how many revolutions have there been a lot? How many dwarf galaxies are there to... Well, two questions. One is how many do we know there are, and how many of them have been studied to the extent that you can use data derived from that observation?

Charlotte Christensen:

Yes. So numbers of dwarf galaxies around the Milky Way I think are on the order of 60 or so right now. I guess I should say [inaudible 00:22:32].

Marshall Poe:

That's pretty reasonable. You can deal with that.

Charlotte Christensen:

Yeah. And recently these surveys have gotten sensitive enough they can see some of these more massive satellite galaxies around more distant spiral galaxies. That's a fairly recent development. One thing that we don't-

Marshall Poe:

These are just the ones around the Milky Way. 60.

Charlotte Christensen:

Let's say in the local group near Andromeda and the Milky Way.

Marshall Poe:

But we would imagine that there are a lot more out there somewhere.

Charlotte Christensen:

Yes. And one thing we don't have much data on are low mass galaxies that are really isolated because they'd have to be far away from us. And these things, because they're low mass, are inherently dim and hard to observe.

Marshall Poe:

Let's talk a little bit about the observation part of it, the actual provision of the data itself. How do you observe a dwarf galaxy? What are the instruments used to do it and what are they measuring?

Charlotte Christensen:

So again, I don't observe these, but-

Marshall Poe:

Right, I was just asking just as-

Charlotte Christensen:

Yeah. But an astronomer, I mean, ideally, if they got their proposal accepted, they would use something like the Hubble Space Telescope or JWST. Those can get really great information. Most of these observations are done in done in optical wavelengths or the visible wavelengths, the type of light that you and I can see with our eyes. Maybe some work is done in the infrared as well. And people use other parts of the spectrum. But those are most important because primarily we're looking at the stars in these galaxies, since they don't tend have a ton of gas.

If an astronomer does want to look at the gas in low mass galaxies, then they might use a radio telescope, for instance, ALMA or the VLA, the Very Large Array, something like that. And then they'd be looking at emission from the gas. So one popular thing to look at would be, it's called a 21 centimeter line, and it's when a hydrogen atom switches from having the spin of the nucleus aligned with the spin of the electron to being anti-aligned. So that was some physics jargon there, but the main point is that hydrogen gas can emit this light and that we would see that using a radio telescope.

Marshall Poe:

And so are the observations fine enough for you to develop a kind of typology of dwarf? Because this is what you'd do in history is you would look at all the revolutions and you'd get all the data you could about them, and then you'd develop a little typology of them.

Charlotte Christensen:

Mm-hmm. So we might characterize these galaxies based off of their mass. We might characterize them by the presence of gas or lack of gas. We characterize them by the rate at which they're currently forming stars, or if they're not forming stars. And we talked about the structure, we might look at the structure. So are these things more elliptical or are they more disc-like?

Marshall Poe:

So this is sort of an unfair question because you've already said this is not an experimental science in the sense of chemistry, let's say. Neither is history, by the way. It is not an experimental science, but we try to give rational explanations for what goes on. How do you know whether your explanation or model is better than another one, absent the possibility of a test?

Charlotte Christensen:

Yeah, I mean, this is one of the difficult things about astronomy.

Marshall Poe:

And history.

Charlotte Christensen:

And history and many other observational sciences. So I think the best thing to do is to make a model based off of the current data set and then make a prediction from that, and then get more data and see if the unknown data matches with that prediction.

Marshall Poe:

I see. So actually then the predictions that you make using the model can be verified or falsified using new data that comes from the galaxy.

Charlotte Christensen:

That is the goal. So for instance, with the launch of JWST, James Web Space Telescope, that's going to allow us to see all sorts of galaxies that haven't yet been studied. And that's going to hopefully verify, I guess, truly falsify or not falsify the models that-

Marshall Poe:

Yeah, I mean, we don't want to get into that whole debate, but, yeah. Right.

Charlotte Christensen:

... that we've developed.

Marshall Poe:

Well, this reminds me much more of what economists do because they produce models as well. And they say, if you raise the interest rate, then this will happen. And then we see if it happens or not, and then they go back and adjust the model, ever tinkering with it. So in the course of your research, was there anything that really surprised you? Was there anything that was like, oh my God, look at that?

Charlotte Christensen:

That's an interesting question, and I'm trying to think the last time I was really surprised. So I'd say one result that I'm pretty fascinated with at the moment, and there was somewhat... that was unexpected, is again, comparing these satellite galaxies to the isolated galaxies. I find that the isolated galaxies just have a slightly lower stellar mass than the satellite galaxies do. And this is counterintuitive because we've talked about how a satellite galaxy-

Marshall Poe:

Yeah, explain. Yeah.

Charlotte Christensen:

Yeah. Next to a Milky Way, it's going to stop forming stars. So why would it have a lower... Why would this satellite galaxy that stopped forming stars have a higher stellar mass than the isolated galaxy which is still forming stars? And that was-

Marshall Poe:

Yeah.

Charlotte Christensen:

Yeah. It's counterintuitive. And that's the puzzle I'm toying with at the moment. And I think it gets down to the timelines over which these things form, that the satellite galaxies are forming earlier on than the isolated ones. And that might be the key to this mystery. I love it when the data is initially counterintuitive that way. You have to rethink the initial assumptions.

Marshall Poe:

Well, yeah, that that's the fun of it, is when you find something unexpected and inexplicable given what you know, and that then that focuses your attention. At least it does in my own research. When I find something that just doesn't look quite right, and then you want to investigate further into that. So is that what you're working on now?

Charlotte Christensen:

Yeah.

Marshall Poe:

This question.

Charlotte Christensen:

That is one of the projects I have going. I've got a few separate ones, but that's what I was working on earlier today and what I will probably work on this afternoon.

Marshall Poe:

Really? Wow. Well, that's great. We've taken up a lot of your time, and I want to thank you very much for being on the show. This has been fascinating for me as a lay person. You're very good at explaining this stuff, by the way. Actually, I feel like I learned a lot. Your students are very lucky.

Charlotte Christensen:

Well, thank you. I mean, I'm at Grinnell because I like explaining this stuff. It's a lot of fun to me.

Marshall Poe:

Yeah. That's good. Well, let me tell everybody that we've been talking to Charlotte Christensen, who is a physicist, actually an astronomer at Grinnell College. And I'm Marshall Poe, the editor of the New Books Network, and the host of Grinnell College's Authors and Artists podcast. Charlotte, thanks so much for being on the show.

Charlotte Christensen:

It's been a pleasure. Thank you.

 

Listen to more episodes of the Grinnell College Authors and Artists Podcast.