The evolution of substructure - I. A new identification method

We describe our new 'MLAPM halo finder' (MHF), which is based on the adaptive grid structure of the N-body code MLAPM. We then extend the MHF code in order to track the orbital evolution of gravitationally bound objects through any given cosmological N-body simulation -our so-called 'MLAPM halo tracker' (MHT). The mode of operation of MHT is demonstrated using a series of eight high-resolution N-body simulations of galaxy clusters. Each of these haloes hosts more than one million particles within their virial radii r vir . We use MHT as well as MHF to follow the temporal evolution of hundreds of individual satellites, and show that the radial distribution of these substructure satellites follows a 'universal' radial distribution irrespective of the environment and formation history of the host halo. This in fact might pose another problem for simulations of cold dark matter structure formation, as there are recent findings by Taylor, Silk & Babul that the Milky Way satellites are found preferentially closer to the Galactic Centre and simulations underestimate the amount of central substructure. Further, this universal substructure profile is anti-biased with respect to the underlying dark matter profile. The halo finder MHF will become part of the open source MLAPM distribution.