Protoclusters in the LCDM universe

We compare the highly clustered populations of very high redshift galaxies with protoclusters identified numerically in a standard Lambda CDM universe (Omega(0) = 0.3, lambda(0) = 0.7) simulation. We evolve 256(3) dark matter particles in a comoving box of side 150 h(-1) Mpc. By the present day there are 63 cluster-sized objects of mass in excess of 10(14) h(-1) M circle dot in this box. We trace these clusters back to higher redshift, finding that their progenitors z = 4-5 are extended regions of typically 20-40 Mpc (comoving) in size, with dark halos of mass in excess of 10(12) h(-1) M circle dot and are overdense by typically 1.3-13 times the cosmological mean density. Comparison with observations of Ly alpha-emitting galaxies at and 4.1 indicates that the observed excess clustering is z p 4.86 consistent with that expected for a protocluster region if Ly alpha-emitters typically correspond to massive dark halos of more than 10(12) h(-1) M circle dot. We give a brief discussion of the relation between the high-redshift concentration of massive dark halos and present-day rich clusters of galaxies.