The X-shaped Milky Way bulge in OGLE-III* photometry and in N-body models

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 αBar ≈30° between the Galactic bar's major axis and the line of sight to theGalactic 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 towards negative longitudes and of the brightness difference between the two red clumps towards positive longitudes, with neither discrepancy exceeding ~25 per cent in amplitude. Our analysis of the red giant luminosity function also yields an estimate of the red giant branch bump parameters towards these high-latitude fields, and evidence for a high rate (~25 per cent) of disc contamination in the bulge at the colour and magnitude of the red clump, with the disc contamination rate increasing towards sightlines further distant from the plane.