This Columbia Astronomy PhD Candidate Is Exploring Why Galaxies Have So Few Stars

Carr's current work explores how galaxies' atmosphere keeps them from forming too many stars.

Christopher D. Shea
March 31, 2023

On the first day of first grade, Chris Carr, now a doctoral candidate in the Department of Astronomy, spotted a book in the library that intrigued him: Venus by Isaac Asimov. He pulled it off the shelf, and ever since then, has been exploring planets and galaxies beyond our own: “In some sense, I haven't really left the library,” he said recently. Carr, who grew up near Cleveland, studied physics and astronomy at Case Western Reserve University as an undergraduate and came to Columbia as a doctoral candidate in 2019. He is particularly interested in dark matter and the question of how galaxies form. In his spare time, he explores Manhattan’s jazz clubs and Korean food offerings. We caught up with Carr to discuss his path to astronomy, his current research, and his favorite things to do in New York City.

What intrigued you about Isaac Asimov’s book, Venus, specifically, and how did it hook you on astronomy?

When I pulled that book off the shelf, I was confronted with these beautiful images of the surface of the planet Venus. You have these clouds of acid, which can melt iron. You have these ancient volcanoes and this cracked, yellowish surface. I think it was that exposure to a world outside of our own, which really catapulted me into the larger universe. Throughout elementary school, I just kept going back to the library and checking out these new books on the planets, the stars, constellations. I've been sold ever since.

Do you have a favorite planet in our solar system, or any other one?

I'm really drawn to Neptune. I think it's often not talked about as much as the other planets, but if you've ever seen Neptune, it's like this emerald jewel out there in space. It’s always attracted me.

What does your current research focus on?

I have a new paper that I’m a lead author on, currently under peer review, which is about how galaxies control their star formation. When astronomers first started to create early models of galaxies, one thing they quickly discovered is that galaxies in the real universe are shockingly inefficient at forming stars: The galaxies that early models of the universe created had orders of magnitude more stars than real galaxies actually have. That suggests that those models were missing something.

This paper looks at a galaxy’s circumgalactic medium, which is basically its atmosphere. We argue that a galaxy’s atmosphere plays a central role in regulating the formation of stars.

What we found with our models is that pockets of dense gas form in a galaxy’s atmosphere, kind of like clouds suspended above the galaxy. If and when those clouds get too dense, they cool very rapidly, and as they cool, they sink, raining fresh gas down onto the galaxy, which provides more fuel for stars to form.

However, once more stars start forming, those stars also eventually die in these spectacular events that we call supernovae, which are some of the most energetic events in the universe. Those events can inject all of this energy out into the galaxy in the form of what are called galactic winds. These winds eject hot gas from inside the galaxy and heat the galaxy’s surrounding atmosphere, preventing the formation of new clouds and ultimately limiting the amount of fresh gas that can rain down onto the galaxy.

So, there’s this constant interplay happening in the galaxy’s atmosphere: between gas cooling and raining onto the galaxy and gas heating from hot galactic winds unleashed by exploding stars. That dance is what keeps star formation in check: It’s the galaxy’s atmosphere that regulates its star formation.

Are you a stargazer?

Funnily enough, not really. New York’s fantastic for stargazing, as you might have heard.

You’ve dabbled in theater during your time at Columbia. Can you tell me about that?

I was part of a workshop at the American Museum of Natural History, which linked together New York astronomers and New York storytellers–professional stand-up comedians, playwrights, actors–to see how the skills and craftsmanship of those professions can inform the way that we talk about science. I had the chance to write my own mini, 10-minute play, which culminated in a staged reading at the museum.

The play I wrote takes place in a cramped New York studio apartment in the summer of 2020. We have a couple that recently went through a really bad breakup. But the day the boyfriend is supposed to move out of the apartment, they both test positive for COVID. It’s about how they navigate being forced into the same place, and having to work through their differences.

Do you have a longstanding interest in writing for the theater?

Growing up, I used to imagine great science fiction space operas in my head and try to put some of them to paper. But those are locked in my brain or my laptop, and probably won’t see the light of day.

What are your favorite things to do in New York?

I love movies. I try to get out to as many movie theaters throughout the city as I can. I also really like jazz. I have a goal of getting to every jazz club in Manhattan at least once. It’s more doable than you might suspect. My favorite jazz clubs are around Greenwich Village, where there’s a nice collection of them. There’s a two-block radius where you have Smalls, Mezzrow, Village Vanguard. It’s probably my favorite pair of streets in the city.

I also like to go out to eat.

What are your favorite places to eat?

My favorite places are definitely in Koreatown. I love Korean food, and I've been to Korea. K- town has always been like this nice, unusual place where you have these restaurants and karaoke bars and tea houses all stacked on top of each other. It kind of transports you. It’s like you're walking the streets of Seoul.