What the Milky Way bulge reveals about the initial metallicity gradients in the disc

Abstract

We examine the metallicity trends in the Milky Way (MW) bulge - using APOGEE DR13 data - and explore their origin by comparing two N-body models of isolated galaxies which develop a bar and a boxy/peanut (b/p) bulge. Both models have been proposed as scenarios for reconciling a disc origin of the MW bulge with a negative vertical metallicity gradient. The first model is a superposition of co-spatial, i.e. overlapping, disc populations with different scaleheights, kinematics and metallicities. In this model the thick, metal-poor, and centrally concentrated disc populations contribute significantly to the stellar mass budget in the inner galaxy. The second model is a single disc with an initial steep radial metallicity gradient, which is mapped by the bar into the b/p bulge in such a way that the vertical metallicity gradient of the MW bulge is reproduced -- as shown already in previous works in the literature. However, as we show here, the latter model does not reproduce the positive longitudinal metallicity gradient of the inner disc, nor the metal-poor innermost regions of the Bulge seen in the data. On the other hand, the model with co-spatial thin and thick disc populations reproduces all the aforementioned trends. We therefore see that it is possible to reconcile a (primarily) disc origin for the MW bulge with the observed trends in metallicity by mapping the inner thin and thick discs of the MW into a b/p. For this scenario to reproduce the observations, the $α$-enhanced, metal-poor, thick disc populations must have a significant mass contribution in the inner regions -- as has been suggested for the Milky Way.