April 15, 2015
When I was 5, the big kid next door gave me “The How and Why Wonder Book of Stars.” I was instantly captivated and stayed that way.
I’m one of the lucky few people for whom Plan A actually worked. I remember my guidance counselor in high school telling me that the number of jobs for astronomers each year was in the tens. I said, “Well, I’ll just have to be one of the top people.”
I worked for 10 years for the Hubble Space Telescope Program and then, along with my wife, Megan Donahue, was recruited to MSU by Steve Zepf in 2003.
Recently, our group at MSU has had a series of breakthroughs in understanding what controls how galaxies form stars. This affects everything we see when we look at the universe.
If you look at all the hydrogen in the universe, about 10 percent is in stars and about 90 percent isn’t. So the universe isn’t very effective at making stars. There are galaxies out there that have plenty of gas, but they’re not making stars. So there must be some type of feedback; once you start making stars, there’s a release of energy that stops more stars from being made. But how does this feedback, this release of energy, which looks like a chaotic mess, lead to a galaxy with very organized properties? That has been a puzzle.
What we found was that while the feedback looks very chaotic, there’s a switch that turns it on and off. And the switch turns out to be very precise. That’s what leads to all these organized properties. In March, we published a paper in Nature about it.
There is a lot of hot gas in galaxy clusters and as it cools it makes many cool gas clouds that keep the central galaxy of the cluster in a perpetual misty state – sort of like galactic rain. Most of those cool clouds go on to form stars. But some of them fall into the black hole at the galaxy’s center and trigger a big chaotic release of energy that temporarily stops the rain of clouds and limits further star formation.
It’s turning out to be a very deep thing. We have submitted another paper to Science that we think is even more important. In the Nature paper, we looked at what regulates star formation in the biggest galaxies in the universe. Then we said, well, if it works there, let’s see if our idea works in galaxies the next size down – massive ellipticals. And the same thing is happening there.
To that point, we had been looking at galaxies in which we could see the hot gas. So the next step was to apply our ideas to even smaller galaxies, where the hot gas couldn’t be seen. If the same “galactic rain” idea held true, then those galaxies should have a certain set of properties. And they do. We were like, “Holy moly – it works!” That’s what the Science paper is about.
The galaxy cluster group at MSU is composed of three physics and astronomy faculty members: Megan Donahue, Brian O’Shea and me. I tell people that Megan does the observations, Brian does the big numerical simulations and I draw cartoons on the whiteboard and pontificate. We each have our strengths. When I took on the role of associate dean for undergraduate education, it forced me to think more big-picture about my research because I didn’t have as much time to spend on it. I think that shift helped lead to these breakthroughs.
We seem to be solving a whole set of outstanding problems with a very simple hypothesis, which is usually the smell of something good in science. So that’s very exciting.
My motivation is puzzle-solving and beauty. If I get into a good puzzle, I have to know the answer. I can’t stop thinking about it. And when I learn something new about how nature works, it’s such a thing of deep beauty that it’s awe-inspiring. The thing that’s most deeply gratifying is the privilege of seeing how the universe works at its most fundamental level.
Photo by Harley Seeley