An Inner Ring and the Microlensing toward the Bulge

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Abstract
All current bulge-disk models for the inner Galaxy fall short of reproducing self-consistently the observed microlensing optical depth by a factor of 2 (>2 σ). We show that the least mass-consuming way to increase the microlensing optical depth is to add density roughly halfway between the observer and the highest microlensing source density. We present evidence for the existence of such a density structure in the Galaxy: an inner ring, a standard feature of barred galaxies. Judging from data on similar rings in external galaxies, an inner ring can contribute more than 50% of a pure bulge-disk model to the microlensing optical depth. We may thus eliminate the need for a small viewing angle of the bar. The influence of an inner ring on the event duration distribution, for realistic viewing angles, would be to increase the fraction of long-duration events toward Baade's window. The longest events are expected toward the negative-longitude tangent point at ℓ ~ -22°. A properly sampled event duration distribution toward this tangent point would provide essential information about viewing angle and elongation of the overall density distribution in the inner Galaxy.