Phase‐Space Distributions of Chemical Abundances in Milky Way–Type Galaxy Halos

Abstract

Motivated by upcoming data from astrometric and spectroscopic surveys of the Galaxy, we explore the chemical abundance properties and phase-space distributions in hierarchically formed stellar halo simulations set in a ΛCDM universe. Our stellar halo metallicities increase with stellar halo mass. The slope of the [Fe/H]-M* trend mimics that of the satellite galaxies that were destroyed to build the halos, implying that the relation propagates hierarchically. All simulated halos contain a significant fraction of old stellar populations accreted more than 10 Gyr ago, and in a few cases some intermediate-age populations exist. In contrast with the Milky Way, many of our simulated stellar halos contain old stellar populations that are metal-rich, originating in the early accretion of massive satellites (M* ~ 109 M☉). We suggest that the (metal-rich) stellar halo of M31 falls into this category, while the more metal-poor halo of the Milky Way is lacking in early massive accretion events. Interestingly, our hierarchically formed stellar halos often have nonnegligible metallicity gradients in both [Fe/H] and [α/Fe]. These gradients extend a few tens of kiloparsecs, and can be as large as 0.5 dex in [Fe/H] and 0.2 dex in [α/Fe], with the most metal-poor halo stars typically buried within the central ~5 kpc of the galaxy. Finally, we find that chemical abundances can act as a rough substitute for time of accretion of satellite galaxies, and, based on this finding, we propose a criterion for identifying tidal streams spatially by selecting stars with [α/Fe] ratios below solar.