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 ( MDF ) decreases as a function of height z from the plane and , more weakly , with galactic radius R _ { GC } .
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 MDF of about —0.45 dex/kpc .
This vertical gradient is believed to reflect the changing contribution with height of different populations in the inner-most 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 MDF 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 the formation and evolution of the Milky Way .