The merger of two heavy black holes was the first event observed through the detection of gravitational waves (GWs), or ripples in spacetime. How these binary black holes (BBHs) form is an important question in astrophysics. One promising scenario is that they form through dynamical interactions of stars in dense stellar clusters (DSCs), in which stars are closer together than in other parts of a galaxy and can frequently interact through their mutual gravity. A research team at Carnegie Mellon University will create the first self-consistent, star-by-star models of dense star clusters with realistic initial conditions taken from cosmological simulations, or simulations of how large groups of stars form from giant clouds of gas. The scientists will then use these models to create a population of the BBH mergers arising in star clusters and also from binaries formed in isolation, using identical initial conditions and stellar physics. The team will then make detailed predictions for future observations from GW detectors such as Advanced LIGO and Virgo, and establish the connection between our understanding of cosmology and galaxy assembly and the new field of gravitational-wave astronomy. This project will also be used to introduce GW astronomy to a larger community. During the course of the research, the lead scientists will work with at least two undergraduate students, who will be supported by summer programs at Carnegie Mellon. The principal investigator (PI) will also continue to visit conferences dedicated to under-represented students in STEM, both to present the work and recruit students.
The researchers will examine several galaxy models created as part of the Feedback in Realistic Environments (FIRE) project, which has recently developed sufficient resolution to resolve the formation and collapse of giant molecular clouds, the progenitors of DSCs. These initial conditions will be integrated forward to the present day using direct N-body and Monte Carlo methods for modeling star clusters, which include all the necessary physics (stellar evolution, gravitational waves, etc.) to predict the present-day appearance of these DSCs and the binary black hole (BBH) mergers that they produce during their evolution. The PI has demonstrated before that there are unique features of BBHs formed in DSCs which will allow them to be distinguished from other formation channels. These features, such as the masses and the orbital eccentricities, depend sensitively on the initial masses, metallicities, and core concentrations of the DSCs. By using the same stellar physics and realistic cosmological initial conditions, this work will show how different galaxy properties, such as the galaxy type and star-formation history influence the gravitational-wave landscape of the local universe. The group plans to make their data public and easily accessible, which will have broad utility in many fields. This project advances the goals of the NSF Windows on the Universe Big Idea.
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.
