Hunting for Black Holes with Astrometric Microlensing

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Black holes are one of the most exotic phenomena in astrophysics and are places where the most fundamental physics: gravity and quantum mechanics are tested. The Milky Way Galaxy likely contains between ten million and a billion stellar-mass black holes: those with masses a few times larger than the Sun. Estimating this number and the true distribution of black hole masses can provide important constraints on how black holes form, the existence of primordial black holes from the early universe, and the interpretation of recent gravitational wave detections of merging black hole pairs. To date, no isolated stellar-mass black holes have been definitively detected and only a few dozen black holes have measured masses -- all in binaries. Microlensing, an effect of general relativity where a black hole's gravity lenses the light of a background star observed from Earth, provides a method to detect isolated black holes and measure their masses. A research team at the University of California-Berkeley will carry out sensitive observations of microlensing events to measure both the magnified brightness and the shifted astrometric position, allowing them to precisely measure the mass of the lensing object and determine if it is indeed a black hole or simply a chance lensing event between two normal stars. The outcome is expected to be the first unambiguous detection of an isolated black hole in the Milky Way. As part of the project, the principal investigator (PI) will also enhance the training and diversity of the next generation of scientists and engineers by improving the training for the teachers who will educate them. The PI will start a U. C. Berkeley chapter of the Institute for Scientist and Engineer Educators, which provides a professional development program that enables junior researchers to receive training in inquiry-based teaching methods and education pedagogy. New inquiry activities developed as a result will be also freely disseminated to a broad community of astronomers to multiply the impact of the program.This project aims to develop a new high-precision astrometry experiment to observe long-duration microlensing events and detect and measure the masses of isolated black holes for the first time. Using astrometric observations obtained from the adaptive optics system on the W. M. Keck Observatory, a sample of approximately seven long-duration microlensing events (i.e. candidate black holes) will be monitored in order to measure their lens masses. The expected accuracy on the lens mass is ~10% for a ten solar mass black hole and the predicted black hole detection rate is ~50%. The resulting black hole detection frequency and masses from the proposed survey will be used to constrain the total number of black holes in the Galaxy, their binary fraction, and kick velocities. Finding these isolated black holes would reduce the orders of magnitude uncertainty on the total number of black holes in the Galaxy and constrain theories of black hole formation and evolution. The proposed development of astrometry techniques, needed for astrometric microlensing, will also lay a foundation for new explorations in many areas of astrophysics.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.