Dark-ages reionization and galaxy formation simulation – II. Spin and concentration parameters for dark matter haloes during the epoch of reionization

We use high-resolution N-body simulations to study the concentration and spin parameters of dark matter haloes in the mass range 108 M⊙ h−1 < M < 1011 M⊙ h−1 and redshifts 5 < z < 10, corresponding to the haloes of galaxies thought to be responsible for reionization. We build a subsample of equilibrium haloes and contrast their properties to the full population that also includes unrelaxed systems. Concentrations are calculated by fitting both NFW and Einasto profiles to the spherically averaged density profiles of individual haloes. After removing haloes that are out of equilibrium, we find a z > 5 concentration–mass (c(M)) relation that is almost flat and well described by a simple power law for both NFW and Einasto fits. The intrinsic scatter around the mean relation is Δcvir ∼ 1 (or 20 per cent) at z = 5. We also find that the analytic model proposed by Ludlow et al. reproduces the mass and redshift dependence of halo concentrations. Our best-fitting Einasto shape parameter, α, depends on peak height, ν, in a manner that is accurately described by α = 0.0070ν2 + 0.1839. The distribution of the spin parameter, λ, has a weak dependence on equilibrium state; λ peaks at roughly ∼0.033 for our relaxed sample, and at ∼0.04 for the full population. The spin–virial mass relation has a mild negative correlation at high redshift.