Microlensing Maps for the Milky Way Galaxy

At any instant, there are roughly 1000 ongoing microlensing events to sourcesbrighter than 20th magnitude in the Milky Way Galaxy. Large-scale maps of themicrolensing optical depth and the mean timescale are constructed for a numberof models of the Galactic bar and disk, incorporating the effects of streamingand spiral structure. Freudenreich's model can reproduce the high opticaldepths towards the Bulge. It is also in good agreement with the data towardsthe spiral arms (except for the gamma Norma field). Spiral structure tends toincrease the optical depth by about 20% and the mean timescale by about 100%.Different bar morphologies give characteristically different shaped contours,especially at low Galactic latitudes (|b| < 2 degrees). These could be tracedout with a K band microlensing survey, consuming about 100 minutes per night ona telescope like VISTA.Comment: 15 pages, 5 figures, ApJ (Letters) in pres