A Dark-Matter Halo Profile allowing a Variable Cusp-Core with Analytic Velocity and Potential

Abstract: We present a useful function for describing the profiles of dark-matter haloes with a varying asymptotic inner slope -\alpha, ranging from a cusp to a core, where the profiles of density, mass-velocity and potential are simple analytic expressions for any \alpha. The idea is to express the mean-density profile as a simple functional form, and obtain the local density by derivative. The model involves a concentration parameter c analogous to the NFW profile. More flexibility is provided by a sum of two such functions, with concentrations c_1 and c_2. An optional additional parameter is the asymptotic outer slope -\gamma, which allows more flexibility in the outskirts of haloes. For \gamma different than 3, there are analytic expressions for the profiles of density and mass-velocity but not for the potential. We match the proposed function, in different variants, to the dark-matter profiles of haloes in cosmological simulations, with and without baryons, spanning inner profiles that range from steep cusps to flat cores, and find excellent fits. The analytic potential profile is useful in modeling the evolution of the inner halo due to episodes of gas inflow and outflow. The analytic profile, especially with a free \gamma, is useful for a study of environmental effects in the outer halo. In general, these analytic profiles can serve for straightforwardly quantifying the shapes of simulated and observed rotation curves, without the need for numerical integrations.