Particle tagging and its implications for stellar population dynamics
  We establish a controlled comparison between the properties of galactic
stellar halos obtained with hydrodynamical simulations and with `particle
tagging'. Tagging is a fast way to obtain stellar population dynamics: instead
of tracking gas and star formation, it `paints' stars directly onto a suitably
defined subset of dark matter particles in a collisionless, dark-matter-only
simulation.Our study shows that there are conditions under which particle
tagging generates good fits to the hydrodynamical stellar density profiles of a
central Milky-Way-like galaxy and its most prominent substructure. Phase-space
diffusion processes are crucial to reshaping the distribution of stars in
infalling spheroidal systems and hence the final stellar halo. We conclude that
the success of any particular tagging scheme hinges on this diffusion being
taken into account, at a minimum by making use of `live' tagging schemes, in
which particles are regularly tagged throughout the evolution of a galaxy.
