The Initial Mass Function of the Inner Galaxy Measured from OGLE-III Microlensing Timescales

We use the timescale distribution of similar to 3000 microlensing events measured by the OGLE-III survey, together with accurate new made-to-measure dynamical models of the Galactic bulge/bar region, to measure the IMF in the inner Milky Way. The timescale of each event depends on the mass of the lensing object, together with the relative distances and velocities of the lens and source. The dynamical model statistically provides these distances and velocities, allowing us to constrain the lens mass function, and thereby infer the IMF. Parameterizing the IMF as a broken power-law, we find slopes in the main-sequence alpha(ms) = 1.31 +/- 0.10|(stat) +/- 0.10|(sys), and brown dwarf region alpha(bd) = -0.7 +/- 0.9|(stat) +/- 0.8|(sys), where we use a fiducial 50% binary fraction, and the systematic uncertainty covers the range of binary fractions 0%-100%. Similarly, for a log-normal IMF we conclude M-c =(0.17 +/- 0.02|(stat) +/- 0.01|(sys)) M-circle dot and sigma(m) = 0.49 +/- 0.07|(stat) +/- 0.06|(sys). These values are very similar to a Kroupa or Chabrier IMF, respectively, showing that the IMF in the bulge is indistinguishable from that measured locally, despite the lenses lying in the inner Milky Way where the stars are mostly similar to 10 Gyr old and formed on a fast alpha-element enhanced timescale. This therefore constrains models of IMF variation that depend on the properties of the collapsing gas cloud.