The ARTEMIS simulations: stellar haloes of Milky Way-mass galaxies
  We introduce the ARTEMIS simulations, a new set of 42 zoomed-in,
high-resolution (baryon particle mass of ~ 2x10^4 Msun/h), hydrodynamical
simulations of galaxies residing in haloes of Milky Way mass, simulated with
the EAGLE galaxy formation code with re-calibrated stellar feedback. In this
study, we analyse the structure of stellar haloes, specifically the mass
density, surface brightness, metallicity, colour and age radial profiles,
finding generally very good agreement with recent observations of local
galaxies. The stellar density profiles are well fitted by broken power laws,
with inner slopes of ~ -3, outer slopes of ~ -4 and break radii that are
typically ~ 20-40 kpc. The break radii generally mark the transition between in
situ formation and accretion-driven formation of the halo. The metallicity,
colour and age profiles show mild large-scale gradients, particularly when
spherically-averaged or viewed along the major axes. Along the minor axes,
however, the profiles are nearly flat, in agreement with observations. Overall,
the structural properties can be understood by two factors: that in situ stars
dominate the inner regions and that they reside in a spatially-flattened
distribution that is aligned with the disc. Observations targeting both the
major and minor axes of galaxies are thus required to obtain a complete picture
of stellar haloes.
