Besan\c{c}on Galactic model analysis of MOA-II microlensing: evidence for a mass deficit in the inner bulge

TL;DR

This study compares microlensing observations from MOA-II with the Besançon Galactic model, revealing a significant mass deficit in the inner bulge likely due to underestimated extinction and star counts.

Contribution

First detailed field-by-field comparison between microlensing data and the Besançon model, including modifications for brown dwarfs and analysis of the inner bulge.

Findings

Model agrees with average event duration

Model underestimates optical depth and event rate by ~50% in inner bulge

Supports existence of a missing inner stellar population

Abstract

Galactic bulge microlensing surveys provide a probe of Galactic structure. We present the first field-by-field comparison between microlensing observations and the Besan\c{c}on population synthesis Galactic model. Using an updated version of the model we provide maps of optical depth, average event duration and event rate for resolved source populations and for difference imaging (DIA) events. We also compare the predicted event timescale distribution to that observed. The simulation follows the selection criteria of the MOA-II survey (Sumi et al. 2013). We modify the Besan\c{c}on model to include M dwarfs and brown dwarfs. Our best fit model requires a brown dwarf mass function slope of $-0.4$. The model provides good agreement with the observed average duration, and respectable consistency with the shape of the timescale distribution (reduced $\chi^2 \simeq 2.2$). The DIA and resolved source limiting yields bracket the observed number of events by MOA-II ($2.17\times$ and $0.83\times$ the number observed, respectively). We perform a 2-dimensional fit to the event spatial distribution to predict the optical depth and event rate across the Galactic bulge. The most serious difficulty for the model is that it provides only $\sim 50\%$ of the measured optical depth and event rate per star at low Galactic latitude around the inner bulge ($|b|<3{^\circ}$). This discrepancy most likely is associated with known under-estimated extinction and star counts in the innermost regions and therefore provides additional support for a missing inner stellar population.