Numerous studies, including small programs and large surveys observing the bulge across a broad spatial extent (e.g. ARGOS, GIBS, VVV) have characterised the overall metallicity distribution of the bulge. Until recently, most spectroscopic studies have looked off the plane, at |b| > 4∘. Although a small number of fields nearer to the plane have been observed (e.g. Zoccali et al. 2014), with the near-IR APOGEE survey (Majewski 2012), the MDF can be examined for the bulge and for regions extending out into the disk, at lower latitudes and in the plane across the entire bulge and into the disk.
Detailed analysis from spectroscopic surveys has also shown the trends of alpha and individual element abundances for stars of the bulge, at all metallicities, to be comparable with those of disk and halo stars of similar metallicity in the local neighbourhood.
Overall the stars of the bulge show similarities to corresponding stars in the nearby disk and halo populations, but there are some critical differences. The [α/Fe] behaviour of bulge stars shown in Fig. 6 illustrates the rapid history of chemical evolution that occurred in the inner Galaxy, probably before the bulge structure was in place. The anomalous scandium abundance of the most metal-poor bulge stars ([Fe/H] ∼ −3.0) may indicate that these very metal-poor stars in the bulge may indeed be among the first stars to have form in the very early Galaxy.
Coupling the stellar metallicity distribution data with the kinematics and morphological structure (Ness et al. 2012; Wegg and Gerhard 2013; Wegg et al. 2015; Portail et al. 2015) is key to understanding the relationship between the bulge and the other populations of the Milky Way that were present at the time that the bulge was formed. Specifically this is important for understanding what fraction of the bulge was formed from pre-existing disk material, and what is the nature of the bulge stars that are not associated with the boxy bulge. Are they simply stars of the disk and inner halo, or is there a unique bulge population that is not related to the disk? A lower limit of ≈ 25% on the mass fraction in the X-structure has been recently determined by Portail et al. (2015). Constraining the total fractional mass of stars in the X-shape is of great interest, as a check on the consistency of the fractional contributions of the different MDF populations with their likely origins.