Formation Models of the Galactic Bulge

TL;DR

This paper reviews formation models of the Galactic bulge, emphasizing N-body simulations and cosmological models that explain its structure, kinematics, and stellar populations as resulting from disk and bar instabilities.

Contribution

It synthesizes recent simulation results and models that account for the Milky Way's bulge features, including its peanut shape and stellar populations.

Findings

Box/peanut bulges arise from disk instabilities in simulations.

Cosmological models can produce bulges with properties similar to the Milky Way.

Bulges can contain both early collapse and later disk instability stellar populations.

Abstract

The Galactic bulge is now considered to be the inner three-dimensional part of the Milky Way's bar. It has a peanut shape and is characterized by cylindrical rotation. In N-body simulations, box/peanut bulges arise from disks through bar and buckling instabilities. Models of this kind explain much of the structure and kinematics of the Galactic bulge and, in principle, also its vertical metallicity gradient. Cosmological disk galaxy formation models with high resolution and improved feedback models are now able to generate late-type disk galaxies with disk-like or barred bulges. These bulges often contain an early collapse stellar population and a population driven by later disk instabilities. Due to the inside-out disk formation, these bulges can be predominantly old, similar to the Milky Way bulge.