Stellar populations of stellar halos: Results from the Illustris
  simulation
  The influence of both major and minor mergers is expected to significantly
affect gradients of stellar ages and metallicities in the outskirts of
galaxies. Measurements of observed gradients are beginning to reach large radii
in galaxies, but a theoretical framework for connecting the findings to a
picture of galactic build-up is still in its infancy. We analyze stellar
populations of a statistically representative sample of quiescent galaxies over
a wide mass range from the Illustris simulation. We measure metallicity and age
profiles in the stellar halos of quiescent Illustris galaxies ranging in
stellar mass from $10^{10}$ to $10^{12} M_\odot$, accounting for observational
projection and luminosity-weighting effects. We find wide variance in stellar
population gradients between galaxies of similar mass, with typical gradients
agreeing with observed galaxies. We show that, at fixed mass, the fraction of
stars born in-situ within galaxies is correlated with the metallicity gradient
in the halo, confirming that stellar halos contain unique information about the
build-up and merger histories of galaxies.
