The large-scale structure of the halo of the Andromeda galaxy. I. Global stellar density, morphology and metallicity properties

Abstract

We present an analysis of the large-scale structure of the halo of the Andromeda galaxy, based on the Pan-Andromeda Archeological Survey (PAndAS), currently the most complete map of resolved stellar populations in any galactic halo. Despite the presence of copious substructures, the global halo populations follow closely power-law profiles that become steeper with increasing metallicity. We divide the sample into stream-like populations and a smooth halo component (defined as the population that cannot be resolved into spatially distinct substructures with PAndAS). Fitting a three-dimensional halo model reveals that the most metal-poor populations () are distributed approximately spherically (slightly prolate with ellipticity c/a = 1.09 ± 0.03), with only a relatively small fraction residing in discernible stream-like structures (f stream = 42%). The sphericity of the ancient smooth component strongly hints that the dark matter halo is also approximately spherical. More metal-rich populations contain higher fractions of stars in streams, with f stream becoming as high as 86% for . The space density of the smooth metal-poor component has a global power-law slope of γ = -3.08 ± 0.07, and a non-parametric fit shows that the slope remains nearly constant from 30 kpc to 300 kpc. The total stellar mass in the halo at distances beyond 2° is 1.1 × 1010 M, while that of the smooth component is 3 × 109 M. Extrapolating into the inner galaxy, the total stellar mass of the smooth halo is plausibly 8 × 109 M. We detect a substantial metallicity gradient, which declines from 〈[Fe/H]〉 = -0.7 at R = 30 kpc to 〈[Fe/H]〉 = -1.5 at R = 150 kpc for the full sample, with the smooth halo being 0.2 dex more metal poor than the full sample at each radius. While qualitatively in line with expectations from cosmological simulations, these observations are of great importance as they provide a prototype template that such simulations must now be able to reproduce in quantitative detail. © 2014. The American Astronomical Society. All rights reserved..