A marked-up image from Professor John Cannon's and student Lila Schisgal's '25 research on a distant dwarf galaxy.

Astronomy, in many ways, is the most pure of the scientific disciplines. Critics might ask, who cares? The stars have nothing to do with everyday life and improving the human experience. And yet, people throughout history, across cultures and time, have gazed up at the stars with wonder and awe. In this context, to study what’s above our heads and so far away really is a unifying quest to satisfy all humans’ innate curiosity about the universe we inhabit. 

Astronomers like Professor John Cannon ask big questions, such as how did our celestial home, the Milky Way, and other huge galaxies form? For Dr. Cannon, who as an observational astronomer employs the use of radio telescopes to conduct his research, the search for answers to such big questions lies in what are called dwarf galaxies.  

What is a dwarf galaxy? 

The question of what a galaxy is, in general, is not all that easy to answer actually. That’s why so many astronomers think about and work on galaxies. Even with many people around the world working with cutting edge telescopes, the question of what is actually a requirement for an object to be called a galaxy is still difficult to answer. 

For the bigger objects like the Milky Way, they’re the most massive things in the universe, and for these galaxies, it’s rather straightforward. But as galaxies get less massive, their properties change. The things that the galaxies do and the way they do them are different. And that’s what intrigues me about these lower-mass objects. And so a dwarf galaxy is ostensibly a galaxy that’s significantly less massive than a big spiral like our own Milky Way, and it has physically different properties. 

In these lower mass galaxies, that is where the in situ physics is happening, where the actual microphysics of the interactions of particles and molecules and gravity and all these things that are happening are allowed to play out. What’s gravity going to do? What’s the energy from stars going to do? And that’s what intrigues me about dwarf galaxies. They have a different kind of physics, and it’s very rich.

How do dwarf galaxies fit into the complex puzzle that is our universe?

That’s a really good question, and if I could answer it with 100 percent certainty, then I would just fly to Sweden and get my medal. But based on what we understand right now, the structures that we see in the universe have formed by a procedure that we describe as hierarchical, meaning that the bigger objects have been built up over cosmic time by smaller objects being gravitationally assimilated, and then we have a bigger, more massive object that now is no longer the original object or the thing that was destroyed. So we think that the Milky Way, as the most common example, has formed by the slow agglomeration of lower mass objects over cosmic time. 

The dwarf galaxies fit into the picture in a very important way. They’re the most numerous kind of galaxy because you need a lot more of them to build up the bigger objects. And if this hierarchical paradigm is correct, then the first objects to gravitationally collapse in the very distant universe are dwarf galaxies. So as we look at those very distant objects, when the universe was younger, we see more of these flocculant, little irregular things. And as we look at objects closer to us physically, we start to see more of those big, organized spirals like the Milky Way. So, the low-mass galaxies fit into the overall picture by being the building blocks that come together over time to build the bigger objects.

What is the focus of your research?

Most fundamentally, I’m interested in understanding the physical characteristics of galaxies. It’s an amazing time to be able to study this discipline because the resources that we have and the instrumentation that’s come online in the last 10 years has really changed the game. If I could describe my work most simply, I try with observation, meaning I use a telescope to acquire data of different kinds, to understand the physical properties of these smaller galaxies. And if we can understand some of them, then, when we make comparisons of those physical properties against those same physical properties in bigger galaxies, we try to understand if the inter-comparisons hold or do not hold. 

Astronomers are very interested in what we often refer to as fundamental correlations. A fundamental correlation is something that describes a system with very basic elemental parameters. For example, a fundamental correlation amongst humans would be that if a human is taller, they must be more massive because there’s more of them. Another fundamental correlation with humans is, if you were to make a graph of the height of humans over time, you’d find that humans sort of asymptote. They grow really fast, and then they kind of stay at a particular height, and then maybe at the end of life they get a little bit shorter again. Those are fundamental relationships. With people, we can do this by taking a census. In order to find out what these fundamental correlations are in astronomy, we have to do it in a different way, where we look out at the different galaxies, and then we try to unpack those parameters and see what’s related and what isn’t, and that’s what my research is focused on.

How do your research findings alter what is known about dwarf galaxies?

When we encode newly understood physical properties of a particular galaxy, that adds to our body of knowledge about what a galaxy is and how it behaves. My work contributes to this knowledge. 

I can give you a concrete example. This past summer, I worked with Macalester student Lila Schisgal ‘25 on a collection of data that we took with a radio telescope in New Mexico. In my research, I identified a couple of galaxies which looked kind of interesting, and so we made a proposal to the National Radio Astronomy Observatory for some time on the telescope to do this experiment, and they gave it to us. When we looked at the data, Lila actually discovered something really important. She found this remarkable structure in one particular galaxy, which was quite literally a three-dimensional hole where gas has been evacuated out of the center for reasons that we’re not really sure of yet. 

To my understanding, this is one of the clearest examples, if not the most clear, of one of these evacuated cavities in the central regions of a low-mass galaxy that I’ve ever seen. This is a result that, as we say, has legs. We’re going to put it out there in the literature and people will see it and say, “Wow. Okay, maybe we need to look at more galaxies to understand if the viewpoint that we had using older data needs to be revised based on the results from this project.” We actually had a paper that was submitted by our group recently that showcases part of these results and Lila is a co-author. 

February 19 2024

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