Microlensing Optical Depth of the COBE Bulge

Abstract

We examine the left-right asymmetry in the cleaned COBE/DIRBE near-infrared data of the inner Galaxy and show (i) that the Galactic bar is probably not seen very nearly end-on, and (ii) that even if it is, it is not highly elongated. The assumption of constant mass-to-light ratio is used to derive simulated terminal-velocity plots for the ISM from our model luminosity distributions. By comparing these plots with observed terminal velocities we determine the mass-to-light ratio of the near-IR bulge and disk. Assuming that all this mass contributes to gravitational microlensing we compute optical depths $τ$ for microlensing in Galactic-centre fields. For three models with bar major axis between $10°-25°$ from the Sun-Galactic Center line, the resulting optical depths in Baade's window lie in the range $0.83\times10^{-6} \lta τ\lta 0.89\times10^{-6}$ for main-sequence stars and $1.2\times10^{-6} \lta τ\lta 1.3\times10^{-6}$ for red-clump giants. We discuss a number of uncertainties including possible variations of the near-infrared mass-to-light ratio. We conclude that, although the values predicted from analyzing the COBE and gas velocity data are inconsistent at the $2-2.5σ$ level with recent observational determinations of $τ$, we believe they should be taken seriously.