remnant of one of such mergers as opposed to one from tidal stirring. Ideally, all these difficulties should be overcome in a fully cosmological hydro simulation of a MW sized galaxy, but with current resolutions around 100 pc and $\sim 10^4 M_{\odot}$ in the baryons numerical results cannot be put on firm grounds for objects that should be only a few hundreds parsecs in size.

Therefore, in order to make progress beyond the idealized simulation we devise a numerical strategy that in the past has only been attempted for simulations of galaxies in clusters ( [@1999ASPC..182..491D]; [@2005MNRAS.364..607M]). In this approach, known as the “replacement technique”, cosmological subhalos are replaced with high resolution models of dwarf galaxies within the original cosmological volume. Models are constructed based on observational and theoretical constraints, and matched as closely as possible to the total virial mass of the parent subhalo. After the replacement the simulation is rerun to the present epoch. In this way galaxy satellites are automatically placed on realistic orbits and are subject to the full tidal interaction history of the parent subhalos. Due to the computational facilities and improved speed and parallelism of the codes, the simulations can span the equivalent of 