Dark-matter halo profiles of a general cusp/core with analytic velocity and potential

We present useful functions for the profiles of dark-matter (DM) haloes with a free inner slope, from cusps to cores, where the profiles of density, mass-velocity and potential are simple analytic expressions. Analytic velocity is obtained by expressing the mean density as a simple functional form, and deriving the local density by differentiation. The function involves four shape parameters, with only two or three free: a concentration parameter c, inner and outer asymptotic slopes a and gamma, and a middle shape parameter beta. Analytic expressions for the potential and velocity dispersion exist for gamma = 3 and for beta a natural number. We match the models to the DM haloes in cosmological simulations, with and without baryons, ranging from steep cusps to flat cores. Excellent fits are obtained with three free parameters (c, a, gamma) and beta = 2. For an analytic potential, similar fits are obtained for gamma = 3 and beta = 2 with only two free parameters (c, a); this is our favourite model. A linear combination of two such profiles, with an additional free concentration parameter, provides excellent fits also for beta = 1, where the expressions are simpler. The fit quality is comparable to non-analytic popular models. An analytic potential is useful for modelling the inner-halo evolution due to gas inflows and outflows, studying environmental effects on the outer halo, and generating halo potentials or initial conditions for simulations. The analytic velocity can quantify simulated and observed rotation curves without numerical integrations.