Simulating Galaxy Formation to Test Dark Matter Theories

by Ted Jou '99

Robyn Sanderson '99 is an Assistant Professor of Physics & Astronomy at the University of Pennsylvania with a joint appointment as an associate research scientist at the Center for Computational Astrophysics at the Flatiron Institute.

Robyn Sanderson
In her research, Sanderson is trying to learn more about the nature of dark matter by using computational simulations and astronomical observations of stars in the Milky Way. These techniques can be traced to the legacy of Vera Rubin, who first observed in the 1970s that the rotational motion of galaxies could not be explained by the gravitational effects of the mass of their stars or other observable objects. This suggested the presence of "dark matter" exerting gravitational force on these galaxies. Direct detection of dark matter has remained elusive - in the decades since Rubin's discovery, physicists have been unsuccessful in their efforts to identify a dark matter particle. But astronomers have continued to follow in Rubin's footsteps, using observations of the movement of stars to make inferences about the nature of dark matter, finding that most galaxies appear to have a "halo" of dark matter affecting their rotation.

Recently, much more information regarding the position and velocity of stars in the Milky Way has become available, and Sanderson is using this data to test different dark matter theories.  In particular, her research compares the observed structure of the Milky Way to simulated versions of the galaxy formed according to different assumptions about the nature of dark matter. This requires expertise working with large astronomy datasets and complex computational models, and Sanderson has experience in both of these areas.

After finishing her Ph.D. at MIT, Sanderson went to the Netherlands to work with Amina Helmi of the Kapteyn Astronomical Institute. Helmi's group was working on mapping the distribution of dark matter in the Milky Way and preparing for the launch of the European Space Agency's Gaia spacecraft, which was designed to catalog over a billion stars in the Milky Way. The data from the Gaia mission was made broadly available to researchers worldwide, and Sanderson had the pleasure of sharing this good news with many American astronomers when she returned to the United States. She received an NSF Postdoctoral Fellowship and started at Columbia University before going west to work with Phil Hopkins (Caltech), who was developing computational models for the formation and development of galaxies.

Sanderson explains that these computational models have become much more sophisticated in recent years, generating results that are consistent with the distribution of stars that we observe in the Milky Way. The initial conditions for these models have also become much more precise, relying on data from the cosmic microwave background obtained by observatories like the Planck satellite. There are still many unknowns, however, including some fundamental questions about the nature of dark matter.

In Sanderson's current research, she has been using simulations of the formation of the Milky Way generated using different assumptions regarding the nature of dark matter. From these models, her group has created "synthetic" Gaia surveys, showing how the galaxy would look if the Gaia mission had been observing one of these hypothetical galaxies. By comparing these synthetic surveys with the actual data from the Gaia mission, Sanderson attempts to evaluate whether certain theories regarding dark matter are more likely to be correct.

A "synthetic" Gaia survey

For Sanderson, being able to compare the results of a mathematical model with real astronomical observations is one of the best parts of doing science. She remembers when in high school she realized how seemingly abstract mathematical concepts could be applied directly to physical phenomena - she was learning about the development of modern physics in Origins of Science and saw the same Calculus concepts in Magnet Analysis. Even now, she finds it "pretty amazing that we can develop mathematical rules that describe the natural world."

She credits the mathematical foundation she received in the Magnet for helping her through college and graduate school. As a teaching assistant for freshman physics at MIT, she was surprised that so many students hadn't received the same kind of high school math education - although they had taken AP Calculus, they had not seen those concepts applied. And she didn't know anyone else that had taken a course in linear algebra in high school, which gave her a head start when learning quantum mechanics.

Sanderson remembers having an interest in astronomy even before she started at Blair, with dreams of being one of the first astronauts to go to Mars. As a Magnet freshman, she made it to the semifinals of a NASA competition with her own proposal for a Mars mission. Opportunities to go to Mars haven't materialized as quickly as she had once hoped, but Sanderson feels lucky to have been able to make her career in astronomy. She remembers being rejected at almost every step of the way, from graduate school to post-doctoral fellowships to getting a faculty appointment, so her advice to others on the academic track is to realize that there isn't only one path for a career in science. As she navigated through different opportunities and rejections, she found that it was important for her own peace of mind to remember that "your options are much more open than you realize."