Cold, Hot, and Feedback: Fundamental Physics of the Circumgalactic Medium

Galaxies are surrounded by large halos of gas that extend far beyond their visible stars. This gas, known as the circumgalactic medium (CGM), contains most of the matter associated with a galaxy. The CGM controls how galaxies grow and form stars. Because this gas is faint and spread over large distances, basic questions about it remain unanswered. This project studies how halo gas changes between cooler and hotter forms, and how energy from stars and black holes affects that gas, helps support the halos and pushes matter outward into intergalactic space. These processes are central to explaining how galaxies fuel star formation and then disperse elements made in stars through the universe. The project advances fundamental knowledge and strengthens U.S. leadership in astrophysics, scientific computing, and data analysis. One graduate student, three undergraduate students, and multiple high school students will be trained in research, numerical modeling, and scientific communication. The team will share results with the public through programs at the Adler Planetarium, Astronomy on Tap events in the Chicago area, Astronomer Evenings at Northwestern's Dearborn Observatory, public lectures, and scientific visualizations. This project uses galaxy formation simulations and comparisons with observations to test competing physical models of the CGM. Work focuses on the transition from cold, photoionized gas to hot, virial-temperature gas, and on the coupling of stellar and active galactic nucleus feedback to halo gas to distinguish between three scenarios for cool CGM gas: compact cool clouds embedded in a hot medium, a volume-filling turbulent cool medium, or a cool medium supported by cosmic-ray pressure. In contrast to classical inside-out accretion shock models, recent simulations predict that CGM virialization proceeds from the outside in due to early feedback heating of the outer halo, implying that the inner CGM remains dominated by turbulent cool gas before it virializes. The project tests this picture and assesses the role of cosmic rays in providing pressure and driving outflows in the CGM. The analysis develops observational diagnostics for each scenario for both cool and hot gas. The project also quantifies where and how feedback deposits energy and momentum in the CGM in different physical regimes. These results will address the origin of high observed OVI columns in the outer CGM of Milky Way-mass galaxies, which suggest an "outer CGM cooling problem," and will provide improved, observation-constrained models of feedback in galaxy formation. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.