Observations show a clear vertical metallicity gradient in the Galactic
bulge, which is often taken as a signature of dissipative processes in the
formation of a classical bulge. Various evidence shows, however, that the Milky
Way is a barred galaxy with a boxy bulge representing the inner
three-dimensional part of the bar. Here we show with a secular evolution N-body
model that a boxy bulge formed through bar and buckling instabilities can show
vertical metallicity gradients similar to the observed gradient, if the initial
axisymmetric disk had a comparable radial metallicity gradient. In this
framework the range of metallicities in bulge fields constrains the chemical
structure of the Galactic disk at early times, before bar formation. Our
secular evolution model was previously shown to reproduce inner Galaxy star
counts and we show here that it also has cylindrical rotation. We use it to
predict a full mean metallicity map across the Galactic bulge from a simple
metallicity model for the initial disk. This map shows a general outward
gradient on the sky as well as longitudinal perspective asymmetries. We also
briefly comment on interpreting metallicity gradient observations in external
boxy bulges.Comment: Accepted to ApJ Letter
