A Possible Dynamical Origin of the Vertical Metallicity Gradient in the Milky Way Bulge

The existence of the vertical metallicity gradient in our Milky Way bulge has been shown clearly by observations. However, this important result has not been well understood in present secular evolution models for the Milky Way. We build a secularly evolved N-body model in which a pure disk self-consistently forms a boxy/peanut bulge via the bar and buckling instability. We find that the vertical metallicity gradient is set up by a two-step mechanism if there exists an initial radial metallicity gradient: the outer (more metal-poor) particles move inward via bar instability and got vertically heated the most in the bar formation and buckling event, than the initially inner particles (more metal-rich). The hotter metal-poor population is distributed in a larger spatial region, and therefore the fraction of the metal-rich population decreases with higher latitude. The mapping between the initial radial gradient and final vertical gradient is linear, indicating that the vertical gradient encodes the galactic dynamical evolution history and the stellar position in the progenitor galaxy. This two-step mechanism for the vertical metallicity gradient must be in action in other more sophisticated disk models, and may constitute an important aspect of the secular evolution. Our simple model can generate the chemo-kinematic relation and asymmetric spatial metallicity distribution of the Galactic bulge. We also find that even such a simple model could result in a multimodal metallicity distribution, which is often regarded as a prediction of the multi-disk scenario. We also discuss the limitations of this simple model.