Inventory of Gravitational Microlenses toward the Galactic Bulge

We determine the microlensing event rate distribution, $\Gamma(t_{E})$, that is properly normalized by the first year MACHO group observations (Alcock et al. 1997). By comparing the determined $\Gamma(t_{E})$ with various MF models of lens populations to the observed distribution, we find that stars and WDs explain just $\sim 49$ of the total event rate and $\sim 37$ of the observationally determined optical depth even including very faint stars just above hydrogen-burning limit. Additionally, the expected time scale distribution of events caused by these known populations of lenses deviates significantly from the observed distribution especially in short time scale region. However, if the rest of the dynamically measured mass of the bulge ($\sim 2.1\times 10^{10} M_\odot$) is composed of brown dwarfs, both expected and observed event rate distribution matches very well.