Exploring dramatic changes in galaxies

The evolution of galaxies over billions of years offers any number of tantalizing clues about the origins of the universe. Alison Coil is trying to solve some of these mysteries by studying how galaxies have been changing over time, and why.

"Galaxies have changed dramatically," says Coil, an associate professor of physics at the University of California, San Diego. "In the past, for example, they formed more stars, and had smaller supermassive black holes. These black holes were more active, brighter and gobbling up materials faster."

The National Science Foundation (NSF)-funded scientist is conducting three research projects with the goal of uncovering some of the physical processes underlying these dramatic changes.

"We are looking at large statistical samples of what galaxies are doing, including nearby galaxies and distant ones," she says.

These include looking at the stellar mass of both nearby and distant galaxies, comparing the properties of galaxies that still are forming stars with those that are not; studying the "clustering" behavior of distant galaxies, that is, the process by which they form a "cosmic web," a filamentary-like structure that resembles a sponge; and quantifying supermassive black holes among distant galaxies.

"This may help us discover if we live in a typical kind of galaxy, and how we came to be, and why we're here," she says. "It tells us something about where we came from. I'd like to understand the galaxy population as a whole, as it helps to put our own Milky Way into context."

Coil is conducting her research under an NSF Faculty Early Career Development (CAREER) award, which she received in 2011. The award supports junior faculty who exemplify the role of teacher-scholars through outstanding research, excellent education, and the integration of education and research within the context of the mission of their organization.

One project is looking halfway back to the Big Bang to compare galaxies still forming stars with those that have stopped. These include nearby galaxies, which formed later and are brighter, as well as distant galaxies, which developed when the universe was younger, and are faint.

"Within the population that is no longer forming stars, there's a lot of growth happening for the lower mass galaxies, they are still getting bigger and there is more of them," she says. "For the population still forming stars, the massive galaxies disappear, and are turning into the other population, those whose star formation has turned off.

"We don't know why," she adds. "We know that one kind turns into the other kind, but we don't know what is shutting off star formation."

Her team also is examining distant galaxies and how they form "cosmic webs," which are clusters of galaxies grouped together like foam, likely the result of gravitational pull.

"The thing that is interesting here is that if you look at the clustering in galaxies still forming stars, and those not forming stars, the clustering is different," she says.

Those galaxies no longer producing stars are more clustered together, and typically are located in the middle of the web, she says.

"Those that are still forming stars tend to be on the outskirts," she says. "We are trying to understand this, whether this has to do with when the galaxies are formed, how much mass they have and whether being near other galaxies shuts off star formation."

Finally, she and her colleagues also are studying supermassive black holes located in the centers of distant galaxies, trying to quantify them.

"We want to know how many galaxies have these super massive black holes, which seem to be fairly common," she says. "They have to be shining for us to see them, and they shine when they are accreting material, that is, when stars and gas fall into the black hole. When you see this light, you know there is a black hole actively accreting. But there are other black holes that exist that we can't see. We want to know why some are shining--accreting--and some aren't."

Much of her work uses data generated by the PRIsm Multi-object Survey (PRIMUS), the largest faint galaxy spectroscopic redshift (which refers to a shift in the spectra of very distant galaxies toward longer wavelengths, usually considered evidence that the universe is expanding) survey taken to date.

As part of the grant's educational component, Coil created and taught a new course during the fall 2013 semester on stars and black holes, and convinced her department to split a pre-existing course entitled "the universe" into two courses, including "stars and black holes," and "galaxies and cosmology."

"The new course is much more fun to teach, as we can spend more time on concepts and class discussions," she says. "I found that the students learned more, at a deeper level. I am very pleased with the new course and hope to attract more students to the lower division astrophysics survey courses as a result."

She also is developing a proposal to add an astrophysics doctoral track in her department, which currently has a physics and a biophysics track. "The astrophysics PhD track would allow students to take more graduate level courses in astrophysics and begin astrophysics research in their first year, a year earlier than the physics PhD track," she says. "I hope to recruit more astrophysics graduate students to UCSD as a result of the new track."

She also informally mentors several female undergraduate and graduate students in the department through individual meetings, and holds monthly meetings of the "women in physics" group she established for women graduate students and postdocs.

During the last year, among other things, the group sponsored two guest speakers--a female faculty in another department and a colloquium speaker who works in science policy--and discussed such topics as diversity in faculty hiring, maternity leave policies, and unconscious bias.

She also runs a one-day physics outreach program for the Reach for Tomorrow foundation, which targets underprivileged middle school youth. About 50 students are participating this year.

Finally, every June she leads a similar program with the Tech Trek program, which is aimed at middle school girls interested in math and science. About 25 girls participate in the physics portion of the program, which involves hands-on demonstrations, and a soldering lab, all run by graduate students and postdocs in the department.

"I specifically recruit mostly women students and postdocs to help run the event, so that the girls can meet and interact with many women who were actively pursuing physics careers," she says.