The Milky Way Bulge: Observed properties and a comparison to external galaxies

TL;DR

This review discusses the properties of the Milky Way bulge, comparing it with external galaxies to understand its formation processes through observations of its stellar populations, structure, and kinematics.

Contribution

It provides a comprehensive comparison of the Milky Way bulge's properties with those of external galaxies, integrating observational data with galaxy formation models.

Findings

Milky Way bulge has a box/peanut shape and possibly an additional spheroidal component.

Chemical abundances and kinematics reveal multiple components in the bulge.

Comparison with external galaxies offers insights into bulge formation scenarios.

Abstract

The Milky Way bulge offers a unique opportunity to investigate in detail the role that different processes such as dynamical instabilities, hierarchical merging, and dissipational collapse may have played in the history of the Galaxy formation and evolution based on its resolved stellar population properties. Large observation programmes and surveys of the bulge are providing for the first time a look into the global view of the Milky Way bulge that can be compared with the bulges of other galaxies, and be used as a template for detailed comparison with models. The Milky Way has been shown to have a box/peanut (B/P) bulge and recent evidence seems to suggest the presence of an additional spheroidal component. In this review we summarise the global chemical abundances, kinematics and structural properties that allow us to disentangle these multiple components and provide constraints to understand their origin. The investigation of both detailed and global properties of the bulge now provide us with the opportunity to characterise the bulge as observed in models, and to place the mixed component bulge scenario in the general context of external galaxies. When writing this review, we considered the perspectives of researchers working with the Milky Way and researchers working with external galaxies. It is an attempt to approach both communities for a fruitful exchange of ideas.