The stellar content of galaxy halos: A comparison between ACDM models and observations of M31

Recent observations have revealed that high surface brightness, metal-rich debris is present over large regions of the Andromeda (M31) stellar halo. We use a set of numerical models to determine whether extended metal-rich debris is expected to exist in galaxy halos formed in a hierarchical ACDM universe. We identify tidal debris in the simulations according to the current best surface brightness detection limits in M31 and demonstrate that bright features in the outer halo regions usually originate from single satellites, with very little contamination from other sources due to the low probability of tidal streams from two overlapping accretion events. In contrast, high surface brightness features in the inner halo often originate from multiple progenitors. We also compare the age and metallicity distribution of the debris with the well-mixed stellar halos in which they reside. We find that high surface brightness tidal debris is produced almost exclusively by relatively high mass progenitors (M* ∼10⁷-10⁹ M_⊙) and thus is expected to be of moderate to high metallicity. Similarly, in our models the smooth inner halo is expected to be metal-rich, as this region has been built up mainly from massive satellites. Our results imply that the stellar populations of substructure observed around external galaxies with current techniques should not resemble old and metal-poor dwarf spheroidal satellites, nor the underlying component of the stellar halo.