CDS&E: Black Holes in Dense Star Clusters

Many stars form in large clusters containing anywhere from thousands to many millions of objects. Stars in these clusters are born with a broad mass spectrum and the most massive ones evolve quickly, ending their lives in just a few million years and leaving behind black holes as remnants. The star clusters themselves, however, can continue to live for many billions of years, and indeed the globular clusters seen in many galaxies are thought to contain some of the oldest stars in the Universe. Therefore, many of the star clusters we see today should contain large numbers of black holes formed a long time ago. Our work will use computer simulations to study the formation and evolution of black holes in a variety of star cluster environments. Intellectual Merit: Through gravitational encounters, black holes are expected to rapidly concentrate in the dense inner cores of clusters, where the rates of dynamical interactions are very high. Such interactions are expected to lead to the formation of many sources of great current interest: bright X-ray binaries studied by NASA's Chandra satellite, and strong gravitational wave sources that should soon become detectable the NSF-funded LIGO. Through emission of gravitational radiation, black holes in tight binaries can ultimately merge: direct evidence for merged, massive black holes, with masses up to thousands of solar masses, is provided by recent detections of many ultraluminous X-ray sources associated with star clusters. Broader Impacts: The study of black hole evolution is important in galaxy formation and cosmology, the study of active galactic nuclei, general relativity and gravitational wave astronomy. The stellar dynamics code to be developed here are general, and will be useful for studying many other problems involving dense star clusters. Some of the novel algorithms and numerical techniques are of wide applicability to problems involving classically interacting particles, even in areas such as plasma physics and nuclear engineering. Our research activities will involve the training of undergraduate students at Northwestern, and will likely include students from under-represented minorities. Graduate students will also receive training and mentoring. Outreach activities are also planned that will take advantage of Dearborn Observatory on the Northwestern campus in Evanston, as well as the resources of the nearby Adler Planetarium and Astronomy Museum in Chicago.