Understanding the Galactic Black Hole Population with Gravitational Microlensing

This thesis addresses the problem of finding and characterizing isolated Galactic stellar-mass BHs via the technique of gravitational microlensing. We present work on both the theoretical and simulation front as well as an observational and modeling perspective.Prior to 2019, most estimates for the number of BHs in the Milky Way were highly uncertain and dated. This limited our ability to accurately predict and interpret the number of BHs detectable with microlensing. To address this shortcoming, we develop a Milky Way microlensing simulation called PopSyCLE (Population Synthesis for Compact object Lensing Events). Using PopSyCLE, we identify an improved strategy for finding and identifying BHs photometrically, advancing our ability to identify good BH candidates and enable statistical constraints on the BH population to be made.Although photometric microlensing can statistically characterize BHs, it cannot disentangle the lensing system’s relative proper motion from the lens’s mass. Astrometric microlensing is required to break this fundamental photometric degeneracy to definitively measure a lens’s mass and confirm its BH nature. We analyze 5 archival BH candidate microlensing events by jointly modeling ground-based microlensing survey photometry and Hubble Space Telescope (HST) astrometric observations. We identify one candidate, OB110462, to be a neutron star or low-mass BH (1.6 - 4.4 solar masses), making it the first dark isolated compact object confirmed with a mass measurement. In addition, using the full sample of five objects including non-BH detections and quantifying selection effects, we find this sample to be consistent with 100 million isolated BHs in the Milky Way.The exact nature of OB110462 was disputed; another group independently found its mass to be 7 solar masses, firmly establishing it as a higher-mass BH. We re-analyze OB110462 using new and re-reduced data, along with updated analysis software. We find that OB110462 is a 6 solar mass BH, making it a solid BH detection. The main reason for finding a higher mass in this work is that the software used in previous work underestimated a positional bias correction, resulting in different astrometric measurements and hence a different mass.Presently, microlensing is a small and relatively obscure subfield of astronomy. However, this will change with the launch of NASA's next flagship mission, the Nancy Grace Roman Space Telescope. Roman will conduct a microlensing survey of the Galactic Bulge. In addition, other public time-domain surveys will enable searches for microlensing events. In order to enable broader participation and newcomers to the field, we provide some examples of important considerations to take into account when modeling microlensing light curves. We also document, explain, and provide examples of some microlensing notations and conventions not easily found in the literature