The Auriga Stellar Haloes: Connecting stellar population properties with
  accretion and merging history
  We examine the stellar haloes of the Auriga simulations, a suite of thirty
cosmological magneto-hydrodynamical high-resolution simulations of Milky
Way-mass galaxies performed with the moving-mesh code AREPO. We study halo
global properties and radial profiles out to $\sim 150$ kpc for each individual
galaxy. The Auriga haloes are diverse in their masses and density profiles;
mean metallicity and metallicity gradients; ages; and shapes, reflecting the
stochasticity inherent in their accretion and merger histories. A comparison
with observations of nearby late-type galaxies shows very good agreement
between most observed and simulated halo properties. However, Auriga haloes are
typically too massive. We find a connection between population gradients and
mass assembly history: galaxies with few significant progenitors have more
massive haloes, possess large negative halo metallicity gradients and steeper
density profiles. The number of accreted galaxies, either disrupted or under
disruption, that contribute 90% of the accreted halo mass ranges from 1 to 14,
with a median of 6.5, and their stellar masses span over three orders of
magnitude. The observed halo mass--metallicity relation is well reproduced by
Auriga and is set by the stellar mass and metallicity of the dominant satellite
contributors. This relationship is found not only for the accreted component
but also for the total (accreted + in-situ) stellar halo. Our results highlight
the potential of observable halo properties to infer the assembly history of
galaxies.
