Modeling the Galactic Bar Using Red Clump Giants

Abstract

The color-magnitude diagrams of ~7 × 105 stars obtained for 12 fields across the Galactic bulge with the Optical Gravitational Lensing Experiment project reveal a well-defined population of bulge red clump giants. We find that the distributions of the apparent magnitudes of the red clump stars are systematically fainter when observing toward lower Galactic l fields. The most plausible explanation for this distinct trend is that the Galactic bulge is a bar whose nearest end lies at positive Galactic longitude. We model this Galactic bar by fitting the observed luminosity functions in the red clump region of the color-magnitude diagram for all fields. We find that, almost regardless of the analytical function used to describe the three-dimensional distribution of stars in the Galactic bar, the resulting models have the major axis inclined to the line of sight from 20° to 30°, with axis ratios corresponding to x0:y0:z0 = 3.5:1.5:1. This puts a strong constraint on the possible range of the Galactic bar models.