Unveiling the Black Hole Populations of Milky Way Globular Clusters

Stellar clusters have emerged as a possible site for black hole binary mergers that would produce gravitational wave sources like those now detected readily by today’s gravitational wave observatories. To test this possible connection, a more robust census of black hole populations and properties in globular clusters is needed. One method for unambiguously identifying a black hole in a globular cluster is via measurements of the radial velocity variation of a luminous stellar companion to the black hole. A research project at the University of California, San Deigo will complete a multi-year program with the Keck Cosmic Web Imager (KCWI) to systematically survey six Galactic globular clusters for new black hole candidates via radial velocity measurements. The program is poised to provide the most robust observational constraints yet on black holes in these dense stellar environments. As part of the project, the researchers will present multimedia shows combining astronomy education and music performance with the goal of making astronomy accessible to the broader public. Motivated by predictions from N-body simulations, that there is a correlation between the size of a globular cluster’s black hole population and its core radius (Rcore), the project will examine three clusters with large observed core radii in the KCWI survey. By performing 1–4 tiled KCWI exposures, complete spatial coverage within 0.25Rcore and extending out to 0.5Rcore will be attained. In total, this yields radial velocity measurements for over a thousand stars in the clusters’ centers. Follow-up observations of candidate binaries already identified from prior MUSE observations in three additional Galactic clusters will also be performed. Given the small number of current detections of black holes in globular clusters, even one new candidate in this parameter space would be significant. As a key complement to the KCWI program, the project will also produce detailed N-body simulations of the six globular cluster targets. This will enable exploration of the key dynamical processes that shape the formation and evolution of stellar and black hole binaries in globular clusters. This is crucial for understanding how observed black hole candidates can constrain the contribution of globular clusters and other dense systems to the catalog of gravitational waves sources observed by LIGO/Virgo/KAGRA. 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.