Modeling Dense Star Clusters from Cosmic Dawn to the Present

Stars are born in dense with a broad range of masses. The most massive ones evolve quickly, ending their lives in as little as a few million years, leaving behind black holes as remnants. The globular clusters seen in all galaxies are thought to contain some of the oldest stars in the Universe. This research team will combinine astronomy, theoretical astrophysics, high-performance computing, and machine learning to develop state-of-the-art computer models of star clusters, simulating their evolution since the earliest stages of star formation in the distant Universe. The focus of this work will be on the dynamical interactions of black holes with other stars, through which they form compact binaries (e.g., two black holes in a tight orbit around each other). These can be strong sources of gravitational waves, as well as high-energy transients detectable in astronomical surveys. This work will also support the principal investigator’s student-oriented research program in computational astrophysics and will contribute to the training of the next generation of US scientists in supercomputing and the use of artificial intelligence and machine learning techniques. Education and public outreach activities will take advantage of the proposing team's close ties to the Adler Planetarium in Chicago and the Dearborn Observatory on the Northwestern campus in Evanston. This project will address several key questions concerning the formation and dynamical evolution of compact objects in dense star clusters, including their strong interactions with other stars. As they experience friction against the background of lighter stars, massive compact remnants are expected to concentrate in the dense inner cores of clusters, where the rates of dynamical interactions are very high. Interactions can then produce binaries, which leads to even higher rates of strong encounters. The electromagnetic transients produced through collisions of compact remnants with other stars potentially include a variety of high-energy burst sources, as well as many optical transients. Additionally, all compact binaries formed through dynamical interactions are strong sources of gravitational waves, of great importance for LIGO and future (ground-based or space-based) detectors. Some of these binaries will have strong electromagnetic counterparts, making them key targets for multi-messenger astronomy. Successive mergers of black holes can increase their mass, perhaps providing the seeds for the formation of supermassive black holes powering quasars at high redshifts. In this context, the investigators will be modeling the earliest star clusters that formed in the Universe at cosmic dawn. 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.