Are the Ogle Microlenses in the Galactic Bar?

TL;DR

This paper analyzes OGLE microlensing data, suggesting that lenses are likely stars in the galactic bar, with a higher-than-expected optical depth, but without evidence linking them to dark matter.

Contribution

It provides observational evidence supporting the hypothesis that galactic bar stars are the primary lenses causing microlensing events in the bulge.

Findings

Optical depth exceeds theoretical estimates.

Lenses are likely stars in the galactic bar.

No evidence linking events to dark matter.

Abstract

The analysis of the first two years of OGLE data revealed 9 microlensing events of the galactic bulge stars, with the characteristic time scales in the range $ 8.6 < t_0 < 62 $ days, where $ t_0 = R_E / V $. The optical depth to microlensing is larger than $ ( 3.3 \pm 1.2 ) \times 10^{-6}$, in excess of current theoretical estimates, indicating a much higher efficiency for microlensing by either bulge or disk lenses. We argue that the lenses are likely to be ordinary stars in the galactic bar, which has its long axis elongated towards us. A relation between $ t_0 $ and the lens masses remains unknown until a quantitative model of bar microlensing becomes available. At this time we have no evidence that the OGLE events are related to dark matter. The geometry of lens distribution can be determined observationally when the microlensing rate is measured over a larger range of galactic longitudes, like $ -10^o < l < +10^o $, and the relative proper motions of the galactic bulge (bar) stars are measured with the HST.