The Metallicity Distribution of the Milky Way Bulge

The Galactic bulge of the Milky Way is made up of stars with a broad range of metallicity, –3.0 < [Fe/H] <1 dex. The mean of the metallicity distribution function decreases as a function of height z from the plane and, more weakly, with galactic radius RGC. The most metal-rich stars in the inner Galaxy are concentrated to the plane and the more metal-poor stars are found predominantly further from the plane, with an overall vertical gradient in the mean of the metallicity distribution function of about −0.45 dex kpc−1. This vertical gradient is believed to reflect the changing contribution with height of different populations in the innermost region of the Galaxy. The more metal-rich stars of the bulge are part of the boxy/peanut structure and comprise stars in orbits which trace out the underlying X-shape. There is still a lack of consensus on the origin of the metal-poor stars ([Fe/H]< −0.5) in the region of the bulge. Some studies attribute the more metal-poor stars of the bulge to the thick disk and stellar halo that are present in the inner region, and other studies propose that the metal-poor stars are a distinct ‘old spheroid’ bulge population. Understanding the origin of the populations that make up the metallicity distribution function of the bulge, and identifying if there is a unique bulge population which has formed separately from the disk and halo, has important consequences for identifying the relevant processes in the formation and evolution of the Milky Way.