Seeing Stars: How postdoctoral fellow William Cramer investigates light in galaxy systems

Author: Madeline Schlehuber

William Cramer

What’s really out there? William Cramer, a postdoctoral research fellow with the Department of Physics and Astronomy, has been asking this question since he was 10 years old, reading all the books about astronomy that his library offered.

“Several rocky physics courses later, I survived.” Survived is one way to put it: Cramer studied physics at University of Chicago, followed that by earning his doctoral degree in astronomy at Yale University, and continued his postdoctoral studies at Arizona State University before coming to the University of Notre Dame this year.

At Notre Dame he is excited to research galaxies with the other astronomers in the department. “A lot of astronomy is broken up into wavelength regimes . . . light can span frequencies from radio, at the low end of the spectrum — all the way to really high, like x-ray. I was mostly doing stuff with radio astronomy. Now I'm more in the optical to infrared range.”

Specifically, Cramer explores what is going on in and around galaxies: the gaseous movement. Gas, explained Cramer, informs researchers about what is going on with the galaxy. Is it old? New? Dying? The gas that moves around galaxies can tell a bigger story about the “health” of the galaxy.

Cramer explained one class of galaxy he has been researching since graduate school: a Ram Pressure Stripped Galaxy. These galaxies live in clusters of other galaxies, surrounded by a hot, gaseous medium. As galaxies move through this medium, the gas within the galaxy can be stripped off and lost. Since galaxies use gas to form new stars, losing all of its gas results in a galaxy slowly dying as stars burn out and are not replenished. At Notre Dame, Cramer is focusing more on the gas around galaxies, called the circumgalactic medium, and how it contributes to the lifecycle of stars and galaxies.

Cramer and the other astronomers interested in these galaxies rely on images for their work.

“All astronomy data is a raw image, and there is a bunch of processing to turn that into what we use for science, using data reduction. It's a process,” Cramer said. The team needs to use programming to analyze the image to find out what makes it special: object brightness, heat, density, phase of matter, and what elements are in the gas.

“It’s not always easy to know what you’ll write a paper about,” he continued. That’s because there are many different things that can show up in telescope images that would alter the direction of his research interests.

As for the images themselves, astronomers get these from telescopes like the James Webb or Hubble, and can propose to have certain areas of the sky imaged. These telescopes capture high-quality images researchers like Cramer use to field advancements.

“I have done some observing — I think I had a really romantic notion of what it would be like, you know, night sky, I would just be sitting there with a telescope,” joked Cramer. “What I should have realized is that you don't want any outside light, so you sit in a windowless room, sealed in from 10 pm to 6 am. It’s a slog. But it’s worth it when you get your images.”

In coming years, Cramer said he hopes to continue his research, and follow in his parents’ footsteps to eventually become a professor. “I take it a year at a time . . . I want to continue to work on cool stuff in the future — I am always proposing to keep looking.”