Halo formation in warm dark matter models

Discrepancies have emerged between the predictions of standard cold dark ma tter (CDM) theory and observations of clustering on subgalactic scales. War m dark matter (WDM) is a simple modification of CDM in which the dark matte r particles have initial velocities due either to their having decoupled as thermal relics or to their having been formed via nonequilibrium decay. We investigate the nonlinear gravitational clustering of WDM with a high-reso lution N-body code and identify a number of distinctive observational signa tures. Relative to CDM, halo concentrations and core densities are lowered, core radii are increased, and large halos emerge with far fewer low-mass s atellites. The number of small halos is suppressed, and those present are f ormed by "top-down" fragmentation of caustics, as part of a "cosmic web" co nnecting massive halos. Few small halos form outside this web. If we identi fy small halos with dwarf galaxies, then their number, spatial distribution , and formation epoch appear in better agreement with the observations for WDM than they are for CDM.