Density profiles of dark matter haloes: diversity and dependence on environment

We study the outer density profiles of dark matter haloes predicted by a ge neralized secondary infall model and observed in a dissipationless cosmolog ical simulation of a low-density flat cold dark matter model with the cosmo logical constant. We find substantial systematic variations in shapes and c oncentrations of the halo profiles as well as a strong correlation of the p rofiles with the environment in which the haloes are embedded. In the N-bod y simulation, the average outer slope of the density profiles, beta (rho pr oportional to r(-beta)), of isolated haloes is beta approximate to 2.9, and 68 per cent of these haloes have values of beta between 2.5 and 3.8. Haloe s in dense environments of clusters are more concentrated and exhibit a bro ad distribution of beta with an average value higher than the average beta for isolated haloes. For haloes located within half the virial radius of th e cluster from the centre values beta approximate to 4 are very common. Con trary to what one may expect, the haloes contained within groups and galaxy systems are less concentrated and have flatter outer density profiles than the isolated haloes: the distribution of beta peaks at approximate to 2.3- 2.7. The slope beta weakly anticorrelates with the halo mass M-h. The conce ntration decreases with M-h, but its scatter is roughly equal to the whole variation of this parameter in the galaxy halo mass range. The mass and cir cular velocity of the haloes are strongly correlated, M-h proportional to V m alpha, with alpha approximate to 3.3 and approximate to 3.5 for the isola ted haloes and haloes in clusters, respectively. For M-h approximate to 10( 12) h(-1) M-circle dot the rms deviations from these relations are Delta lo g M-h = 0.12 and 0.18, respectively. Approximately 30 per cent of the haloe s are contained within larger haloes or have massive companions within thre e virial radii. The companions are allowed to have masses larger than simil ar to 0.3 times the mass of the current halo. The remaining 70 per cent of the haloes are isolated objects. We find that the distribution of beta as w ell as the concentration-mass and M-h-V-m relations for the isolated haloes agree very well with the predictions of our seminumerical approach, which is based on a generalization of the secondary infall model and on the exten ded Press-Schechter formalism.