Mary Putman Is Investigating the Fuel That Powers Stars

Putman, a professor of astronomy, recently won an award for “high risk, high reward” research on our galaxy.

Christopher D. Shea
November 13, 2023

Mary Putman studied astronomy at the University of Wisconsin, in her home state, before decamping to Australia for graduate school. There, for four years, she studied the views of the night sky visible from the Southern Hemisphere, which offer many treasures for young scientists to explore, since that hemisphere has traditionally had fewer telescopes focused on it, and fewer astronomers examining its data. Putman returned to the United States for a Hubble postdoctoral fellowship at the University of Colorado and then went on to teach at the University of Michigan before coming to Columbia, where she’s taught since 2008.

Now, Putman often focuses on dwarf galaxies and the question of how, over the course of millions of years, free-floating gas in the universe gets drawn into galaxies, where it becomes the fuel for stars to form. Putman and her colleagues have recently been awarded a Cottrell Plus SEED (Singular Exceptional Endeavors of Discovery) Awards for “high risk, high reward,” as well as a grant from the National Science Foundation. Columbia News caught up with her to discuss the winning projects, and her work more broadly.

What’s the main focus of your research right now?

I look at how gas moves across the universe. In particular, I look at the gas that’s outside of galaxies, in what we call the “intergalactic medium.” I examine how that gas flows into the “circumgalactic medium,” which is the region surrounding a galaxy, and eventually down into the galaxy itself, where it becomes the fuel that feeds the formation of stars. I primarily use radio telescopes and data from the Hubble Space Telescope, and I also do a fair amount of comparing real data with data simulations.

I also look at very small galaxies, known as dwarf galaxies. These galaxies are interesting because they’re a source of fuel for larger galaxies, which often absorb them. They’re also interesting because they are the first galaxies to have formed in the universe.

In one recent paper I worked on with a student, Jingyao Zhu, we looked at the dwarf galaxies close to our own to assess how they’re losing their gas. What happens is that when these dwarf galaxies get closer to larger neighboring galaxies, they experience something a bit like when you stick your head out of a car and get wind in your hair that blows it back: The larger galaxy blows aways the smaller galaxy’s gas. Mind you, this is not happening at human time scales, but over the course of millions of years.

You were just awarded a Cottrell Plus SEED (Singular Exceptional Endeavors of Discovery) Award for “high risk, high reward” research. Can you tell us about the project that won that award?

This project came from the original work of a graduate student of mine, Susan Clark, who is now on faculty at Stanford, and who will be working with me and one of our graduate students, Avery Kim.

Susan mapped filaments of gas in our Milky Way galaxy, and she found that those filaments were aligned with the Milky Way’s magnetic field. Magnetic fields are very important in terms of star formation: In galaxies, there are clouds of dust and gas that would be crushed down by gravity were it not for the presence of magnetic fields.

This new project that I’m working on will look at nearby galaxies, with the idea of mapping out magnetic fields in those galaxies. The broader goal here is to understand more about the origin of magnetic fields, which we see across the universe, but whose root cause and source is still unclear. Hopefully with this project we can start to build a model of how the arrangement and orientation of filaments of gas in galaxies across the universe relates to those galaxies’ magnetic fields, and it will lead us to answers to bigger questions.

You and astronomy chair Greg Bryan also recently got a grant from the National Science Foundation. Can you explain that research?

The idea here is that any galaxy, including our own, needs fuel to keep forming stars.  And there’s always been a bit of a mystery because it seems like our galaxy would run out of fuel, but we don’t. With this new grant, we’re going to be comparing new observations of the clouds around the Milky Way to high resolution simulations that Bryan and his students work on. The goal is to get a sense of whether these clouds of gas will be destroyed as they move down into the galaxy, or whether they’ll actually grow, which is what some simulations suggest will happen. Our goal is that by comparing direct observations to simulations, we will get a better understanding of what is actually happening to these clouds of gas that prevents them from slowly disappearing.

You grew up in the U.S. and got your PhD in Australia. How did you decide to study there?

I finished my undergrad in the middle of a school year, in December, so I was thinking of ways to spend half a year abroad when I finished, before starting graduate school. I sent out a bunch of emails asking universities if they had temporary research positions overseas, and the Australian National University responded, and asked if I had considered their graduate program. I looked and found that there were really good people there, and I applied and got full funding to attend. I just decided to go for it.

The southern sky—the bits of the universe that you can see from Australia, and from the Southern Hemisphere generally—is a bit of a gem, because it’s traditionally been less studied, though that’s changing somewhat with big sky surveys like the one that Columbia is involved in that’s based out of Chile.

There’s lots of fascinating stuff that you can see down South, like the Magellanic Clouds, a pair of dwarf galaxies that look like two little smudges to the naked eye, so it was a great place to study.

I did my work primarily at Mt. Stromlo observatory, which is outside of Canberra on a large hill, away from the main campus of Australia National University. I really enjoyed mountain biking around the hill, dodging kangaroos late at night.

Do you still do work in Australia?

I’m just starting to do that again, actually. One idea for radio astronomy in the future is to have a “square kilometer array,” which means a square kilometer of radio dishes that act together as one huge dish. Both South Africa and Australia have had these prototypes and they’re now collecting survey data. So I’m diving back into that new data now.

Do you have any favorite hobbies in New York, when you're not working? 

My hobbies are now dominated by my 8-year-old. Coaching kids soccer and cheering on Columbia's basketball teams are two of the big ones.