Detecting and directly measuring masses of isolated stellar remnants, especially neutron stars and black holes, is virtually impossible with traditional astrophysical methods. Our knowledge of the mass function of neutron stars and black holes is based on observations of binary systems but the binary evolution likely affects the final mass of the compact object. However, isolated neutron stars and black holes must ubiquitous in our Galaxy. Knowledge of their mass function would give us important clues about the evolution of massive stars, core collapse and supernova mechanisms, etc.
I will present the results of the study of a sample of over 15,000 gravitational microlensing events detected during the third and fourth phases of the OGLE survey. I estimated the masses of lensing objects by combining photometric data from OGLE and proper-motion information from OGLE and Gaia EDR3. I then selected high-probability dark lenses — white dwarfs, neutron stars, and black holes — which I used to measure the mass function of isolated stellar remnants. I will discuss how the shape of the remnant mass function depends on the environment (disk, bulge) and compare OGLE observations with the results from LIGO and Virgo and predictions of population synthesis models of black hole formation.