The formation of dark halos in a universe dominated by cold dark matter

Abstract

We investigate the formation of galactic halos in a flat universe dominated by cold dark matter. The amplitude and scale of the initial fluctuations are set to agree with previous models for the large-scale galaxy distribution. We use gravitational N-body simulations to study cubic regions of present size ~14 Mpc and of varying density. We follow such regions from z = 6 to the present with a linear resolution which decreases from ~2 to ~14 kpc. Halos of galactic scale form abundantly in these models only after z ~ 3. Their early evolution is dominated by mergers, but the merger rate begins to decline after z ~ 1. Although recent mergers are responsible for the most massive of present-day halos, many smaller systems grew in a relatively quiescent fashion after z ~ 0.5. Throughout the central regions of most dark halos the circular velocity is almost independent of radius, predicting a "rotation curve" similar in form to those of spiral galaxies. However, the circular velocities of the most massive systems rise significantly between 50 and 100 kpc. We conjecture that these objects correspond to the massive halos of elliptical galaxies or to the merged halos of small groups. The bulk rotation velocity of halos is typically only about 10% of their velocity dispersion and is roughly independent of radius. Many halos are strongly aspherical with three unequal axes. There is, perhaps, a slight preference for near-prolate shapes. The mass distribution of the halo population is well approximated by a power law of index -2 with a high mass cutoff which grows rapidly with time, especially in high-density regions. If we identify the cumulative mass distribution of dark halos with the cumulative luminosity distribution of galaxies, we are able to reproduce both the form and the amplitude of the Tully-Fisher/Faber-Jackson relation. This match between the predicted abundance of halos and the observed abundance of galaxies is a major success of the model. Massive halos form preferentially in regions of high dark matter density and are thus naturally biased relative to the mass. Their formation sites correspond well to high peaks of the initial linear density field. This bias may be as strong as is required to reconcile a flat universe with the dynamics of observed galaxy systems, but further work is needed to demonstrate this point conclusively. Our models set the stage for galaxy formation in the cold dark matter cosmogony; they suggest explanations for the different morphological types of galaxies, for their differing distributions, and for indications of substantial evolution in galaxy properties at moderate redshift.