ar X iv : a st ro - p h / 98 03 03 0 v 1 4 M ar 1 99 8 The seeds of rich galaxy clusters in the Universe

The discovery[1] of a population of young galaxies at an epoch when the universe was about one tenth of its current age has shed new light on the question of when and how galaxies formed. Within the context of popular models[2] this is the population of primeval galaxies that built themselves up to the size of present–day galaxies through the process of repeated mergers. But the recent detection [3] 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 two powerful 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 understanding of galaxy formation within the framework of a universe dominated by cold dark matter. The longstanding search for primeval galaxies was recently brought to fruition by the combination of deep imaging and Keck Telescope spectroscopy [4]. Because they were originally identified by a sharp break in their spectrum, at the wavelength corresponding to the limit of the Lyman series of hydrogen, these galaxies are called " Lyman break " galaxies. The most distant examples have redshift z ≃ 3.5, corresponding to the time when the universe was only about 10% of its current age. The structure discovered by Steidel and collaborators[3] appears as a spike in the redshift distribution at z = 3.09. It contains 15 Lyman-break galaxies in a 9 × 18 arcminute field [3]. The comoving spatial dimensions of this structure are huge, approximately 8×10×13h −1 Mpc in a Universe with critical density. (Throughout, we denote Hubble's constant as H 0 = 100hkms −1 Mpc −1 .) The current paradigm for the formation of cosmic structure is the cold dark matter theory [5]. For suitable values of the parameters, this theory provides a good description of large-scale structure, from scales probed by microwave background anisotropy measurements to those mapped by galaxy surveys [6]. In this theory, galaxies form by gas cooling and condensing into dark matter halos which grow by accretion and mergers in a hierarchical fashion. There …