Voronoi tessellation and non-parametric halo concentration

We present and test Tessellation-based Recovery of Amorphous halo Concentrations (TesseRACt), a non-parametric technique for recovering the concentration of simulated dark matter halos using Voronoi tessellation. TesseRACt is tested on idealized N-body halos that are axisymmetric, triaxial, or contain substructure and is compared to traditional least-squares fitting as well as to two non-parametric techniques that assume spherical symmetry. TesseRACt recovers halo concentrations within 3% of the true value regardless of whether the halo is spherical, axisymmetric, or triaxial. Traditional fitting and non-parametric techniques that assume spherical symmetry can return concentrations for non-spherical halos that are systematically off by as much as 10% from the true value. TesseRACt also performs significantly better when there is substructure present outside 0.5 R₂₀₀. Given that cosmological halos are rarely spherical and often contain substructure, we discuss implications for studies of halo concentration in cosmological N-body simulations including how choice of technique for measuring concentration might bias scaling relations.