Formation of the Galactic Halo

TL;DR

This paper reviews the formation processes of the Galactic stellar halo, highlighting the roles of dissipative collapse and accretion, and compares observational data with models to understand their relative contributions.

Contribution

It provides a comprehensive analysis of halo formation, integrating observational evidence with advanced models that include feedback, challenging previous simple relations and supporting hierarchical formation.

Findings

Accretion accounts for about 10% of the halo.

Halo kinematics show evidence for both formation processes.

Models with feedback better match observed distributions.

Abstract

Recent observational and theoretical work suggests that the formation of the Galactic stellar halo involved both dissipative processes and the accretion of subfragments. With present data, the fraction of the halo for which an accretion origin can be substantiated is small, of order 10 percent. The kinematics of the best halo field star samples show evidence for both dissipative and dissipationless formation processes. Models of star-forming dissipative collapse, in a cosmological context and including feedback from star formation, do not confirm the simple relations between metallicity, rotation velocity, and orbital eccentricity for halo stars as originally predicted. The new model predictions are much closer to the observed distributions, which have generally been interpreted as evidence for an accretion origin. These results are broadly consistent with a hierarchical galaxy formation model, but the details remain to be worked out.