Solving the Small Scale Crisis of Galaxy Formation

Ordinary matter holds the key to understanding our Galactic origins

Most of the matter in our Universe resides in “dark matter.” There is nearly six times as much dark matter in the Universe than the ordinary matter that we are made of, and that makes up the visible part of galaxies like stars. For decades it has been assumed that, because there is so much more dark matter than ordinary matter, dark matter dominates the gravity in the Universe, and that wherever the dark matter is, ordinary matter must follow. This faulty assumption led theorists to make predictions for the formation of galaxies that have led to a number of discrepancies between galaxy formation theory and observations. These discrepancies have evaded solution for so many years that they have become known collectively as the “small scale crisis” of the Cold Dark Matter (CDM) model for galaxy formation.

Dr. Alyson Brooks, Assistant Professor of Physics and Astronomy at Rutgers University, is an observationally-oriented theoretical astrophysicist who uses high-resolution simulations of galaxies run on national supercomputers to make sense of the small scale crisis. With the hope of addressing the bigger question of “where did we come from?” her simulations explain how our galaxy formed and evolved and how we came to be located in this unique structure. Simulations include both ordinary matter (gas and stars) and dark matter and span the entire age of the Universe, roughly 13.7 billion years. Using sophisticated tools and measurement, Dr. Brooks has made an important discovery: gas and stellar physics can alter the dark matter structure of galaxies. This discovery has changed the view of galaxy formation with Dr. Brooks and her colleagues at the forefront of this revolution. In short, she and her team have shown how ordinary matter can alter the observed structure of galaxies from the older, simpler picture, a task that no scientist was able to do despite almost two decades of trying.

Current research includes:

  • Bulgeless disk galaxies lack a central concentration of stars. Supernovae (exploding stars) drive winds that naturally eject low angular momentum gas from the centers of galaxies, preventing stellar bulge formation. By developing the simulations that show the mechanisms that lead to bulgeless disks, Dr. Brooks and her collaborators are the first ever to simulate bulgeless disk galaxies.

  • The processes that drive galaxy winds also create fluctuations in the gravitational potential wells of galaxies that push dark matter out of central regions. Dr. Brooks’ simulation of this problem reconciles observations of the central regions of galaxies that had previously been at odds with predictions from theory.

  • Supernova explosions perturb the gravitational potential well of the galaxy, changing the orbits of both dark matter and stars. This completely reshuffles the stars in “dwarf” galaxies that are roughly 1/100 the mass of our Milky Way. Dr. Brooks is demonstrating how this reshuffling explains recent and ongoing observations with the Hubble Space Telescope that previously defied easy explanation.

Dr. Brooks’ work in galaxies is currently also the best way to constrain the allowed physics of the dark matter particle. She is working with particle physicists to test models of self-interacting dark matter. In this model, dark matter particles can exchange a dark "photon" with other dark matter particles. Dr. Brooks is running simulations that include self-interacting dark matter, as well as the full physics of gas and stars, and placing new bounds on the properties of dark matter.


At eight years old, Dr. Alyson Brooks decided she wanted to be an astronomer. Each summer, at her family reunion campout, aunts, uncles, cousins, and grandparents would turn to her to ask questions about constellations or planets. Despite her enthusiasm for space and the unknowns contained within it, as she grew older she questioned whether a career in the sciences would satisfy her.

Therefore, as a college freshmen, Dr. Brooks enrolled as an English major having decided to pursue other interests. Despite her full course load, she also enrolled in an astronomy class during her first semester. Dr. Brooks was fortunate that at her small undergraduate university she was able to discover a young, intelligent, woman astronomer who immediately became her role model. Shortly after, Dr. Brooks changed her major to physics with a minor in astronomy and began working on real astronomy projects. From then on, she was hooked.

When admitted to graduate school, Dr. Brooks expected to be an observational astronomer. However, after a few unexpected project assignments, she learned that she had a passion for theory. Since her early career in astronomy, Dr. Brooks has remained a theorist and enjoys working on projects that look at the world in a new or original way.

Aside from research, Dr. Brooks spends her additional time with her young daughter, exercising, and indulging her passion for animated films.


2015 Alfred P. Sloan Research Fellow in Physics

Grainger Fellowship, U Wisconsin - Madison, 2011-2013

Sherman Fairchild Fellowship in Theoretical Astrophysics, Caltech, 2008-2011

Washington Space Grant Consortium Fellow, 2002-2003, and Spring 2008

Sigma Xi Grant-in-Aid of Research, 2005