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

Understand

We use the timescale distribution of ~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 provides statistically these distances and velocities allowing us to constrain the lens mass function, and from this to infer the IMF.
• Parameterising the IMF as a broken power-law, we find slopes in the main sequence $\alpha_{\rm ms}=1.31\pm0.10|_{\rm stat}\pm0.10|_{\rm sys}$ and brown dwarf region $\alpha_{\rm bd}=-0.7\pm0.9|_{\rm stat}\pm0.8|_{\rm sys}$ where we use a fiducial 50% binary fraction, and the systematic uncertainty covers the range of binary fractions 0-100%.