The stellar halo of the Galaxy

TL;DR

This review discusses the structure, kinematics, and chemistry of the Milky Way's stellar halo, highlighting recent discoveries of substructure and the prospects for understanding its formation through upcoming surveys and Gaia data.

Contribution

It synthesizes current knowledge on the Galactic halo's properties and explores formation scenarios in light of recent observational breakthroughs.

Findings

Detection of extensive substructure in the outer halo

Implications for galaxy formation models

Future prospects with Gaia and large surveys

Abstract

Stellar halos may hold some of the best preserved fossils of the formation history of galaxies. They are a natural product of the merging processes that probably take place during the assembly of a galaxy, and hence may well be the most ubiquitous component of galaxies, independently of their Hubble type. This review focuses on our current understanding of the spatial structure, the kinematics and chemistry of halo stars in the Milky Way. In recent years, we have experienced a change in paradigm thanks to the discovery of large amounts of substructure, especially in the outer halo. I discuss the implications of the currently available observational constraints and fold them into several possible formation scenarios. Unraveling the formation of the Galactic halo will be possible in the near future through a combination of large wide field photometric and spectroscopic surveys, and especially in the era of Gaia.