A “Light,” centrally concentrated milky way halo?

Abstract

We discuss a novel approach to "weighing" the Milky Way (MW) dark matter halo, one that combines the latest samples of halo stars selected from the Sloan Digital Sky Survey (SDSS) with state of the art numerical simulations of MW analogs. The fully cosmological runs employed in the present study include "Eris," one of the highest resolution hydrodynamical simulations of the formation of a M vir = 8 × 1011 M ⊙ late-type spiral, and the dark-matter-only M vir = 1.7 × 1012 M ⊙ "Via Lactea II" (VLII) simulation. Eris provides an excellent laboratory for creating mock SDSS samples of tracer halo stars, and we successfully compare their density, velocity anisotropy, and radial velocity dispersion profiles with the observational data. Most mock SDSS realizations show the same "cold veil" recently observed in the distant stellar halo of the MW, with tracers as cold as σlos ≈ 50 km s-1 between 100 and 150 kpc. Controlled experiments based on the integration of the spherical Jeans equation as well as a particle tagging technique applied to VLII show that a "heavy" M vir ≈ 2 × 1012 M ⊙ realistic host produces a poor fit to the kinematic SDSS data. We argue that these results offer added evidence for a "light," centrally concentrated MW halo.