Cosmological simulations of dark matter halos

We perform a series of cosmological simulations in which we model the formation and evolution of dark matter halos on scales ranging from dwarfs to clusters. We analyze the evolution of the structural properties of 16 ΛCDM halos of ∼106 particles each, with masses ranging from dwarfs to clusters, including a group with 7 million particles. 10 of these halos have been extracted from a 50 h−1Mpc volume containing 80 million particles, which we use to study bulk statistical halo properties, including the mass function and clustering properties. Halo density profiles have a large cosmological scatter in their central slopes bounded approximately on either extreme by r−1 of Navarro, Frenk, & White (1997, NFW) and r −1.5 of Moore et al. (1999, M99) profile slopes. The characteristic density of our halos increases for lower mass and lower redshift halos, worsening the discrepancy with the flat, low density cores inferred from dwarf LSB rotation curves. We simulate a cluster with power law fluctuation spectra ( P ∝ kn) for a range of n, finding that the central slope has a weak dependence on spectral index. None of our simulated halos have inner slopes flatter than the NFW central slope of r−1 down to our minimum resolved radii of ∼0.5% to ∼2%. The velocity distribution function of subhalos is nearly universal and scales approximately as V _circular−4, though there is evidence that it declines at higher redshift, and also a hint of increased substructure in lower mass halos at redshift zero. The Sheth & Tormen analytic mass function is an excellent match to our simulated mass function, except for an overprediction at redshifts higher than 10. The friends-of-friends group finding algorithm commonly used in optical group catalogs is able to recover the dark matter mass function from a subhalo catalog reasonably well above ∼1013h−1$M\odot$, despite low completeness, high contamination, and errors of a factor of roughly 3 or more in the mass estimates of individual halos. We compare halo clustering properties to recent results from 2dF and SDSS redshift surveys. Additionally, we also explore the possibility that the density fluctuation spectrum may be suppressed on the group scale, where it is still poorly constrained by optical, X-ray, and CMB data. A detailed analysis of the numerical uncertainties in our highest resolution halos, performed by modeling identical halos at incrementally lower resolution, reveals no evidence of any systematic bias in our density profiles or substructure analysis.