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

TL;DR

This paper demonstrates that a secular evolution N-body model of the Milky Way's bar and buckling instabilities can produce a vertical metallicity gradient in the bulge similar to observations, linking it to the initial disk's chemical structure.

Contribution

It introduces a simple model showing that metallicity gradients in the bulge can arise from bar dynamics, challenging the idea they solely indicate dissipative formation processes.

Findings

The model reproduces observed vertical metallicity gradients.

It predicts a full metallicity map with asymmetries across the bulge.

The model exhibits cylindrical rotation consistent with observations.

Abstract

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.