The survival of subhaloes in galaxies and galaxy clusters
  This paper discusses physical and numerical disruption processes acting on subhaloes in galaxy haloes and in galaxy cluster haloes, and compare the effects of these processes on the subhalo abundance within both types of haloes. N-body simulations with a resolution high enough not to suffer from overmerging (subhalo disruption due to numerical processes) show a high abundance of subhaloes in both galaxies and in galaxy clusters. However, observations seem to show a high subhalo abundance in galaxy clusters only. Thus, it appears that too many subhaloes survive in simulated galaxy haloes. There are five main causes for this apparent galaxy subhalo problem. The most radical one is a change to a cosmology in which structure formation is not hierarchical below the galaxy halo mass scale. If this is unacceptable, four causes remain, of which the most important one appears to be that dynamical friction is not properly simulated yet, not even for the highest resolution simulations to date, resulting in an 'undermerging' problem. The other causes are (numerical) overmerging, differences in the timing of halo formation and merging in hierarchical structure formation, and significant differences is mass-to-light ratios. The net effect of these four causes is that galaxies have a relatively low abundance of subhaloes, i.e. dwarfs, while at the same time a large number of field dwarf galaxies can exist which are dark enough to be missed observationally. 
Eelco van Kampen (IfA, Edinburgh)