Microlensing towards different Galactic targets

TL;DR

This paper calculates microlensing optical depths and event rates towards various Galactic targets, estimates stellar mass functions, and explores the influence of the Magellanic Clouds on halo shape and microlensing observations.

Contribution

It provides new estimates of the stellar mass functions in the bulge and disk using microlensing data and analyzes the halo shape effects due to the Magellanic Clouds through N-body simulations.

Findings

Mass function follows a power-law with slope ~2.0 in bulge and disk.

Expected microlensing events in spiral arms agree with observations.

Magellanic Clouds cause halo flattening and reduce expected halo microlensing events by ~20%.

Abstract

We calculate the optical depth and the number of events due to gravitational microlensing towards the Galactic bulge, the spiral arm directions $\gamma$ Scutum, $\beta$ Scutum, $\gamma$ Normae, $\vartheta$ Muscae and some dwarf galaxies in the halo of the Galaxy. Using the events found by the MACHO collaboration during their first year of observation towards Baade's Window we estimate the mass functions for the bulge and disk populations following the mass moment method. We find that the mass function can be described by a decreasing power-law with slope $\alpha \simeq 2.0$ in both cases and a minimal mass of $\sim 0.01$ $M_{\odot}$ for the bulge and $\sim 0.02$ $M_{\odot}$ for the disk, respectively. Assuming that the obtained mass function for the disk is also valid in the spiral arms, we find that the expected number of events towards the spiral arms is in reasonable agreement with the observations. However, the small number of observed events does not yet constrain much the different parameters entering in the computation of the mass function. To study the influence of the Magellanic Clouds on the shape and the velocity dispersion in the halo we perform a N-body simulation. We find that their presence induces a slight flattening of the halo ($q_{H}\simeq 0.8$). As a result the expected number of microlensing events towards some targets in the halo, such as the LMC or the SMC, decreases by about 20%, whereas due to the the modification induced on the velocity dispersion the event duration increases.