The 3D structure of the Galactic bulge

TL;DR

This paper reviews the three-dimensional structure of the Galactic bulge, emphasizing its bar-like shape, boxy/peanut features, and the distinct properties of metal-poor stars, providing a comprehensive overview of observational evidence.

Contribution

It synthesizes current observational data to clarify the complex structure of the Galactic bulge, highlighting the bar, its shape variations, and the role of metal-poor stars.

Findings

The bulge is dominated by a bar structure.

The bar exhibits a boxy/peanut shape in outer regions.

Metal-poor stars trace a different, less elongated structure.

Abstract

We review the observational evidences concerning the three-dimensional structure of the Galactic bulge. Although the inner few kpc of our Galaxy are normally referred to as {\it the bulge}, all the observations demonstrate that this region is dominated by a bar, i.e., the bulge is a bar. The bar has a boxy/peanut (X\--shaped) structure in its outer regions, while it seems to become less and less elongated in its innermost region. A thinner and longer structure departing from the main bar has also been found, although the observational evidences that support the scenario of two separate structures has been recently challenged. Metal poor stars ([Fe/H]$\lesssim -0.5$ dex) trace a different structure, and also have different kinematics.