A cosmological context for compact massive galaxies

To provide a quantitative cosmological context to ongoing observational work on the formation histories and location of compact massive galaxies, we locate and study a sample of exceptionally compact systems in the BOLSHOI simulation, using the dark matter structural parameters from a real, compact massive galaxy (NGC 1277) as a basis for our working criteria. We find that over 80 per cent of objects in this nominal compact category are substructures of more massive groups or clusters, and that the probability of a given massive substructure being this compact increases significantly with the mass of the host structure; rising to similar to 30 per cent for the most massive clusters in the simulation. Tracking the main progenitors of this subsample back to z = 2, we find them all to be distinct structures with scale radii and densities representative of the population as a whole at this epoch. What does characterize their histories, in addition to mostly becoming substructures, is that they have almost all experienced below-average mass accretion since z = 2; a third of them barely retaining, or even losing mass during the intervening 10 Gyr.