The seeds of rich galaxy clusters in the Universe

The discovery1 of a population of young galaxies at a redshift when the Universe was about a tenth of its current age has shed new light on the question of when and how galaxies formed. Within the context of popular models2, this is the population of primeval galaxies that built themselves up to the size of present-day galaxies through the process of repeated mergers called hierarchical clustering. But the recent detection3 of a large concentration of these primeval galaxies appears to be incompatible with hierarchical clustering models, which generally predict that clusters of this size are fully formed later in time. Here we use a combination of theoretical techniques — semi-analytic modelling and n-body simulations — to show that such large concentrations should be quite common in a universe dominated by cold dark matter, and that they are the progenitors of the rich galaxy clusters seen today. We predict the clustering properties of primeval galaxies which should, when compared with data that will be collected in the near future, test our current understanding of galaxy formation within the framework of a universe dominated by cold dark matter.