An inner ring and the microlensing toward the bulge

All current bulge-disk models for the inner Galaxy fall short of reproducin g self-consistently the observed microlensing optical depth by a factor of 2 (>2 sigma). We show that the least mass-consuming way to increase the mic rolensing optical depth is to add density roughly halfway between the obser ver and the highest microlensing source density. We present evidence for th e existence of such a density structure in the Galaxy: an inner ring, a sta ndard feature of barred galaxies. Judging from data on similar rings in ext ernal 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 nee d for a small viewing angle of the bar. The influence of an inner ring on t he event duration distribution, for realistic viewing angles, would be to i ncrease the fraction of long-duration events toward Baade's window. The lon gest events are expected toward the negative-longitude tangent point at l s imilar to -22 degrees. A properly sampled event duration distribution towar d this tangent point would provide essential information about viewing angl e and elongation of the overall density distribution in the inner Galaxy.