Concentration of a dark-matter halo ($c$)

The concentration of a spherical dark-matter halo compares its outer radius with an inner scale radius. For a Navarro--Frenk--White profile, $c=r_v/r_s$.

The mass-concentration relation describes the statistical dependence of halo concentration on virial mass and epoch. In hierarchical cold-dark-matter cosmologies, lower-mass haloes generally formed earlier against a denser background and are therefore more concentrated at fixed epoch.