Early Formation and Late Merging of the Giant Galaxies

The most luminous galaxies in the present Universe are found at the centersof the most massive dark matter haloes, rich galaxy clusters. In the LCDMcosmology, such massive halo cores are present at redshift z=6 with a comovingnumber density (as a function of mass interior to ~10 kpc) that is comparableto today's value. The identity of the matter in these central regions is,however, predicted to change as major mergers bring together stars and darkmatter from initially well separated sub-units. We use N-body simulations toinvestigate how these mergers push pre-existing matter outwards in the dominantgalaxy while preserving the inner density profile of collisionless matter. Itappears that the central regions of large galaxies end up dominated by starsformed in a number of dense cores, well before the last major mergers. Thedensity profile of collisionless matter (stars and dark matter combined) inthese central regions appears to be stable and to have attractor-like behaviorunder merging. This suggests that the baryon loading associated withdissipative contraction and star formation may be erased as subsequent mergersdrive the mass distribution back to a universal profile. Such suppression ofthe effects of baryon loading, along with the early assembly of massconcentrations, may help resolve some apparent challenges to the CDM model forstructure formation.Comment: 25 pages, 9 figures, acctepted by ApJ. Minor changes in tex