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

TL;DR

This study models the formation of the Milky Way bulge's vertical metallicity gradient through a two-step heating process involving bar and buckling instabilities, highlighting its significance in secular galaxy evolution.

Contribution

It introduces a novel 'two-step heating' mechanism in N-body models that explains the origin of the vertical metallicity gradient in the Galactic bulge.

Findings

The 'two-step heating' mechanism effectively transforms radial into vertical metallicity gradients.

Models show the importance of bar and buckling instabilities in bulge formation.

The mechanism is consistent with the formation of boxy/peanut-shaped bulges.

Abstract

A vertical metallicity gradient 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 for each disk. These models generate a vertical metallicity gradient through a ``two-step heating" mechanism: first 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 radial metallicity gradients in precursor disks into vertical metallicity gradients. 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 vertical metallicity gradient. 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.