We introduce the ARTEMIS simulations , a new set of 42 zoomed-in , high-resolution ( baryon particle mass of \approx 2 \times 10 ^ { 4 } { M } _ { \odot } / 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 \approx - 3 , outer slopes of \approx - 4 and break radii that are typically \approx 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 .