Dynamical Origin of the Vertical Metallicity Gradient of the Milky Way Bulge

Abstract A vertical metallicity gradient (VMG) in the Milky Way bulge is well-established. Yet, its origin has not been fully understood under the Galactic secular evolution scenario. We construct single-disk and triple-disk N -body models with an initial radial metallicity gradient (RMG) for each disk. These models generate a VMG through a “two-step heating” mechanism: the outer, metal-poor particles move inward via the bar instability and subsequently undergo more significant vertical heating during the buckling instability, so they end up at greater vertical height. The “two-step heating” mechanism nearly linearly transforms the RMGs in precursor disks into VMGs. Comparing the models with a triple-disk model tagged with radially independent Gaussian metallicity, we find that, despite certain limitations, the “two-step heating” mechanism is still important in shaping the Galactic VMG. If the bar and buckling instabilities contributed to the formation of boxy/peanut-shaped bulges, then the “two-step heating” mechanism is inevitable in the secular evolution of a boxy/peanut-shaped bulge.