Theoretical Models of the Galactic Bulge

TL;DR

This paper reviews the evolving understanding of the Milky Way's bulge, emphasizing its likely origin as a bar structure formed from disk instabilities rather than classical mergers, supported by various observational evidence.

Contribution

It summarizes current theoretical models and techniques for understanding the Galactic bulge, highlighting the shift from merger-based to disk instability formation scenarios.

Findings

The bulge is likely a bar seen end-on, not a classical merger remnant.

Cylindrical rotation and X-shaped structure support the bar formation model.

A complete formation model explaining all observations remains elusive.

Abstract

Near infrared images from the COBE satellite presented the first clear evidence that our Milky Way galaxy contains a boxy shaped bulge. Recent years have witnessed a gradual paradigm shift in the formation and evolution of the Galactic bulge. Bulges were commonly believed to form in the dynamical violence of galaxy mergers. However, it has become increasingly clear that the main body of the Milky Way bulge is not a classical bulge made by previous major mergers, instead it appears to be a bar seen somewhat end-on. The Milky Way bar can form naturally from a precursor disk and thicken vertically by the internal firehose/buckling instability, giving rise to the boxy appearance. This picture is supported by many lines of evidence, including the asymmetric parallelogram shape, the strong cylindrical rotation (i.e., nearly constant rotation regardless of the height above the disk plane), the existence of an intriguing X-shaped structure in the bulge, and perhaps the metallicity gradients. We review the major theoretical models and techniques to understand the Milky Way bulge. Despite the progresses in recent theoretical attempts, a complete bulge formation model that explains the full kinematics and metallicity distribution is still not fully understood. Upcoming large surveys are expected to shed new light on the formation history of the Galactic bulge.