Microlensing optical depth and event rate toward the Galactic bulge from
  8 yr of OGLE-IV observations
  The number and properties of observed gravitational microlensing events
depend on the distribution and kinematics of stars and other compact objects
along the line of sight. In particular, precise measurements of the
microlensing optical depth and event rate toward the Galactic bulge enable
strict tests of competing models of the Milky Way. Previous estimates, based on
samples of up to a few hundred events, gave larger values than expected from
the Galactic models and were difficult to reconcile with other constraints on
the Galactic structure. Here we used long-term photometric observations of the
Galactic bulge by the Optical Gravitational Lensing Experiment (OGLE) to select
a homogeneous sample of 8000 gravitational microlensing events. We created the
largest and most accurate microlensing optical depth and event rate maps of the
Galactic bulge. The new maps ease the tension between the previous measurements
and Galactic models. They are consistent with some earlier calculations based
on bright stars and are systematically ~30% smaller than the other estimates
based on "all-source" samples of microlensing events. The difference is caused
by the careful estimation of the source star population. The new maps agree
well with predictions based on the Besancon model of the Galaxy. Apart from
testing the Milky Way models, our maps may have numerous other applications,
such as the measurement of the initial mass function or constraining the dark
matter content in the Milky Way center. The new maps will also inform the
planning of future space-based microlensing experiments by revising the
expected number of events.
