THE STRUCTURE OF COLD DARK-MATTER HALOS

We use N-body simulations to investigate the structure of dark halos i n the standard cold dark matter cosmogony. Halos are excised from simu lations of cosmologically representative regions and are resimulated i ndividually at high resolution. We study objects with masses ranging f rom those of dwarf galaxy halos to those of rich galaxy clusters. The spherically averaged density profiles of all our halos can be fitted o ver two decades in radius by scaling a simple ''universal'' profile. T he characteristic overdensity of a halo, or equivalently its concentra tion, correlates strongly with halo mass in a way that reflects the ma ss dependence of the epoch of halo formation. Halo profiles are approx imately isothermal over a large range in radii but are significantly s hallower than r(-2) near the center and steeper than r(-2) near the vi rial radius. Matching the observed rotation curves of disk galaxies re quires disk mass-to-light ratios to increase systematically with lumin osity. Further, it suggests that the halos of bright galaxies depend o nly weakly on galaxy luminosity and have circular velocities significa ntly lower than the disk rotation speed. This may explain why luminosi ty and dynamics are uncorrelated in observed samples of binary galaxie s and of satellite/spiral systems. For galaxy clusters, our halo model s are consistent both with the presence of giant arcs and with the obs erved structure of the intracluster medium, and they suggest a simple explanation for the disparate estimates of cluster core radii found by previous authors. Our results also highlight two shortcomings of the CDM model. CDM halos are too concentrated to be consistent with the ha lo parameters inferred for dwarf irregulars, and the predicted abundan ce of galaxy halos is larger than the observed abundance of galaxies. The first problem may imply that the core structure of dwarf galaxies was altered by the galaxy formation process, and the second problem ma y imply that galaxies failed to form (or remain undetected) in many da rk halos.