Microlensing Optical Depth Revisited with Recent Star Counts

TL;DR

This study refines constraints on the Galactic microlensing optical depth by comparing models based on recent star count surveys with observational data, highlighting the importance of accurate Galactic models for understanding microlensing phenomena.

Contribution

It evaluates and compares Galactic bulge and disk models against microlensing data, identifying models that best fit the observations and suggesting directions for future observations.

Findings

Models based on 2MASS and certain previous studies agree well with observations.

Models involving SDSS disk data show higher chi^2, indicating poorer fit.

Discrepancies increase at low Galactic latitudes, near the Galactic center.

Abstract

More reliable constraints on the microlensing optical depth comes from a better understanding of the Galactic model. Based on well-constrained Galactic bulge and disk models constructed from survey observations, such as, HST, 2MASS, and SDSS, we calculate the microlensing optical depths toward the Galactic bulge fields, and compare them with recent results of microlensing surveys. We test chi^2 statistics of microlensing optical depths expected from those models, as well as previously proposed models, using two types of data: optical depth map in (l, b) and averaged optical depth over the Galactic longitude l as a function of the latitude b. From this analysis, we find that the Galactic bulge models of 2MASS, Han & Gould (2003), and G2 of Stanek et al. (1997) show a good agreement with the microlensing optical depth profiles for all the microlensing observations, compared with E2 of Stanek et al. (1997). We find, on the other hand, that models involving an SDSS disk model produce relatively higher chi^2 values. It should be noted that modeled microlensing optical depths diverge in the low Galactic latitude, |b| < 2 arcdeg. Therefore, we suggest the microlensing observation toward much closer to central regions of the Galaxy to further test the proposed Galactic models, if it is more technically feasible than waiting for large data set of microlensing events.