An accurate comprehensive approach to substructure - I. Accreted subhaloes

Abstract

This is the first of a series of three papers devoted to the study of halo substructure in hierarchical cosmologies by means of the CUSP formalism. In the present paper, we derive the properties of subhaloes and diffuse dark matter (dDM) accreted on to haloes and their progenitors. Specifically, we relate the dDM present at any time in the inter-halo medium of the real Universe or a cosmological simulation with the corresponding free-streaming mass or the halo resolution mass, respectively, and establish the link between subhaloes and their seeds in the initial density field. By monitoring the collapse and virialization of haloes, we derive from first principles and with no single free parameter the abundance and radial distribution of dDM and subhaloes accreted on to them. Our predictions are in excellent agreement with the results of simulations, but for the predicted fraction of accreted dDM, which is larger than reported in previous works as they only count the dDM accreted on to the final halo, not on to its progenitors. The derivation pursued here clarifies the origin of some key features of substructure. Overall, our results demonstrate that CUSP is a powerful tool for understanding halo substructure and extending the results of simulations to haloes with arbitrary masses, redshifts, and formation times in any hierarchical cosmology endowed with random Gaussian density perturbations.