Resolving the Structure of Cold Dark Matter Halos

We study the effects of mass and force resolution on the density profiles ofgalaxy-size Cold Dark Matter (CDM) halos in a flat, low-density cosmologicalmodel with vacuum energy. We show thatalthough increasing the mass and forceresolution allows us to probe deeper into the inner halo regions, it does notlead to steeper inner density profiles. Instead, the halo profiles converge atscales larger than four times the formal resolution or the radius containingmore than 200 particles, whichever is larger. In the simulations presented inthis paper, we are able to probe density profile of a relaxed isolatedgalaxy-size halo at scales r=(0.005-1)r_vir. We find that the densitydistribution can be well approximated by the profile suggested by Moore etal(1998): rho= x^{-1.5}(1+x^{1.5})^-1, where x=r/r_s and r_s is thecharacteristic radius. The analytical profile proposed by Navarro et al. (1996)rho= x^{-1}(1+x)^-2, also provides a good fit, with the same relative errors ofabout 10% for radii larger than 1% of the virial radius. Both analyticalprofiles fit equally well because for high-concentration galaxy-size halos thedifferences between these profiles become significant only at scales well below0.01r_vir. We also find that halos of similar mass may have somewhat differentparameters (characteristic radius, maximum rotation velocity, etc.) and shapesof their density profiles. We associate this scatter in properties withdifferences in halo merger histories and the amount of substructure present inthe analyzed halos.Comment: substantially revised, accepted to ApJ, 19 pages, 9 figures, uses aastex and natbi