Tracing Galaxy Formation with Stellar Halos II: Relating Substructure in Phase- and Abundance-Space to Accretion Histories

Abstract

This paper explores the mapping between the observable properties of a stellar halo in phase- and abundance-space and the parent galaxy's accretion history in terms of the characteristic epoch of accretion and mass and orbits of progenitor objects. The study utilizes a suite of eleven stellar halo models constructed within the context of a standard LCDM cosmology. The results demonstrate that coordinate-space studies are sensitive to the recent (0-8 Gyears ago) merger histories of galaxies (this timescale corresponds to the last few to tens of percent of mass accretion for a Milky-Way-type galaxy). Specifically, the {\it frequency, sky coverage} and {\it fraction of stars} in substructures in the stellar halo as a function of surface brightness are indicators of the importance of recent merging and of the luminosity function of infalling dwarfs. The {\it morphology} of features serves as a guide to the orbital distribution of those dwarfs. Constraints on the earlier merger history (> 8 Gyears ago) can be gleaned from the abundance patterns in halo stars: within our models, dramatic differences in the dominant epoch of accretion or luminosity function of progenitor objects leave clear signatures in the [alpha/Fe] and [Fe/H] distributions of the stellar halo - halos dominated by very early accretion have higher average [alpha/Fe], while those dominated by high luminosity satellites have higher [Fe/H]. This intuition can be applied to reconstruct much about the merger histories of nearby galaxies from current and future data sets.