Synthetic microlensing maps of the Galactic bulge

International audienceWe present synthetic maps of the microlensing optical depth, event rate and time-scales over an area of 195 deg2 towards the Galactic bulge. The maps are computed from stellar population synthesis catalogues generated from the Besançon Galaxy Model, which comprises four stellar populations and a three-dimensional extinction map calibrated against the Two-Micron All-Sky Survey. The microlensing maps have a resolution of 15 arcmin, corresponding to the angular resolution of the extinction map. We compute optical depth and event rate maps for all resolved sources above I= 19, for unresolved ‘difference image’ (DIA) sources magnified above this limit, and for bright ‘standard candle’ sources. The resulting optical depth and rate contours are dominated by extinction effects, in contrast to previous theoretical microlensing maps, and underscore the need to include realistic extinction modelling. Comparison of optical depth measurements to the Besançon model favours a bulge mass of ∼1.3 × 1010 M⊙. The microlensing maps exhibit a systematically higher optical depth for DIA sources, in line with experimental determinations. The predicted contours of the microlensing rate show very good correspondence with the field positions of the OGLE-III survey, illustrating how they have been optimized to maximize microlensing yields. We also show how the event distribution for hypothetical J and K-band microlensing surveys, using existing ground-based facilities such as VISTA, UKIRT or CFHT, would be much less affected by extinction, especially in the K band. The near infrared provides a substantial sensitivity increase over current I-band surveys and a more faithful tracer of the underlying stellar distribution, something which upcoming variability surveys such as Vista Variables in the Via Lactea (VVV) will be able to exploit. Only a fraction of current data sets has been employed for Galactic structure studies and more detailed models will be required to fully exploit the full catalogue of current survey samples, not to mention those coming online in the near future. Synthetic population modelling offers a promising way forward towards high-precision Galactic structure studies using microlensing