Microlensing constraints on the Galactic Bulge Initial Mass Function

TL;DR

This study uses microlensing event durations and optical depth profiles to constrain the initial mass function of the Galactic bulge, finding slopes consistent across datasets but limited by data constraints.

Contribution

It provides new estimates of the bulge IMF slopes using microlensing data and compares observed and expected optical depths, highlighting data limitations.

Findings

IMF slope for main sequence stars: α=1.7 ± 0.5

Consistent slopes across EROS, MACHO, OGLE datasets

Data insufficient to distinguish bulge models

Abstract

Aims. We seek to probe the Galactic bulge IMF starting from microlensing observations. Methods. We analyse the recent results of the microlensing campaigns carried out towards the Galactic bulge presented by the EROS, MACHO and OGLE collaborations. In particular, we study the duration distribution of the events. We assume a power law initial mass function, $\xi(\mu)\propto \mu^{-\alpha}$, and we study the slope $\alpha$ both in the brown dwarf and in the main sequence ranges. Moreover, we compare the observed and expected optical depth profiles. Results. The values of the mass function slopes are strongly driven by the observed timescales of the microlensing events. The analysis of the MACHO data set gives, for the main sequence stars, $\alpha=1.7 \pm 0.5$, compatible with the result we obtain with the EROS and OGLE data sets, and a similar, though less constrained slope for brown dwarfs. The lack of short duration events in both EROS and OGLE data sets, on the other hand, only allows the determination of an \emph{upper} limit in this range of masses, making the overall result less robust. The optical depth analysis gives a very good agreement between the observed and the expected values, and we show that the available data do not allow one to discriminate between different bulge models.