The X-Shaped Milky Way Bulge in OGLE-III Photometry and in N-Body Models

TL;DR

This study models the split red clump in the Galactic bulge using OGLE-III data and compares it with N-body models, revealing insights into the bulge's structure and the models' accuracy.

Contribution

It provides detailed mapping of red clump features and evaluates the accuracy of existing N-body models in predicting bulge characteristics.

Findings

Models predict the red clump fraction variation with longitude.

Discrepancies are within 25% for surface density and brightness differences.

Evidence of significant disk contamination in the bulge region.

Abstract

We model the split red clump of the Galactic bulge in OGLE-III photometry, and compare the results to predictions from two N-body models. Our analysis yields precise maps of the brightness of the two red clumps, the fraction of stars in the more distant peak, and their combined surface density. We compare the observations to predictions from two N-body models previously used in the literature. Both models correctly predict several features as long as one assumes an angle $\alpha_{\rm{Bar}} \approx 30^{\circ}$ between the Galactic bar's major axis and the line of sight to the Galactic centre. In particular that the fraction of stars in the faint red clump should decrease with increasing longitude. The biggest discrepancies between models and data are in the rate of decline of the combined surface density of red clump stars toward negative longitudes and of the brightness difference between the two red clumps toward positive longitudes, with neither discrepancy exceeding $\sim$25% in amplitude. Our analysis of the red giant luminosity function also yields an estimate of the red giant branch bump parameters toward these high-latitude fields, and evidence for a high rate ($\sim$25%) of disk contamination in the bulge at the colour and magnitude of the red clump, with the disk contamination rate increasing toward sightlines further distant from the plane.