Gravitational Microlensing and the Structure of the Inner Milky Way

Abstract

We analyze the first-year MACHO collaboration observations of microlensing towards the Galactic center using a new direct likelihood technique that is sensitive to the distribution of the events on the sky. We consider the full set of 41 events, and calculate the direct likelihood against a simply-parameterized Galactic model consisting of either a gaussian or exponential bar and a double exponential disk. Optical depth maps are calculated taking into account the contribution of both disk lenses and sources. We show that based on the presently available data, a slope in the optical depth has been clearly detected ($3σ$) in Galactic latitude and that there are indications of a small slope in Galactic longitude. We discuss limits that can be set on the mass, angle and axis ratio of the Galactic bulge. We show that based on microlensing considerations alone, $M_{Bulge}>1.5\times 10^{10}\Msol$ at the 90\% confidence level and that the bulge inclination angle is less than $30 °$ also at the 90\% confidence level. The mostly likely bar mass is $M_{Bulge}=3.5\times10^{10}\Msol$. Such a high mass would imply a low MACHO fraction for the halo. We consider disk parameters and show that there are two degeneracies between the effects of a disk and those of a bar on the optical depths. Finally, we discuss how to break these degeneracies and consider various strategies for future microlensing observations.