Origins of stellar halos

Abstract

This contribution reviews ideas about the origins of stellar halos. It includes discussion of the theoretical understanding of and observational evidence for stellar populations formed in situ (meaning formed in orbits close to their current ones), kicked-out (meaning formed in the inner galaxy in orbits unlike their current ones) and accreted (meaning formed in a dark matter halo other than the one they currently occupy). At this point there is general agreement that a significant fraction of any stellar halo population is likely accreted. There is modest evidence for the presence of a kicked-out population around both the Milky Way and M31. Our theoretical understanding of and the observational evidence for an in situ population are less clear.