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Observations show a clear vertical metallicity gradient in the Galacticbulge, which is often taken as a signature of dissipative processes in theformation of a classical bulge. Various evidence shows, however, that the MilkyWay is a barred galaxy with a boxy bulge representing the innerthree-dimensional part of the bar. Here we show with a secular evolution N-bodymodel that a boxy bulge formed through bar and buckling instabilities can showvertical metallicity gradients similar to the observed gradient, if the initialaxisymmetric disk had a comparable radial metallicity gradient. In thisframework the range of metallicities in bulge fields constrains the chemicalstructure of the Galactic disk at early times, before bar formation. Oursecular evolution model was previously shown to reproduce inner Galaxy starcounts and we show here that it also has cylindrical rotation. We use it topredict a full mean metallicity map across the Galactic bulge from a simplemetallicity model for the initial disk. This map shows a general outwardgradient on the sky as well as longitudinal perspective asymmetries. We alsobriefly comment on interpreting metallicity gradient observations in externalboxy bulges.

Metallicity gradients through disk Instability: A simple model for the Milky Way's boxy bulge