Dark-ages Reionization & Galaxy Formation Simulation II: Spin and concentration parameters for dark matter haloes during the Epoch of Reionization

Paul W. Angel, Gregory B. Poole, Aaron D. Ludlow, Alan R. Duffy, Paul M. Geil, Simon J. Mutch, Andrei Mesinger, J. Stuart B. Wyithe

Published 2015-12-02Version 1

We use high resolution N-Body simulations to study the concentration and spin parameters of dark matter haloes in the mass range $10^8\, {\rm M}_{\odot}\, h^{-1} < {\rm M} < 10^{11}\, {\rm M}_{\odot}\, h^{-1}$ and redshifts $5{<}z{<}10$, corresponding to the haloes of galaxies thought to be responsible for reionization. We build a sub-sample 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 $\Delta c_{\rm{vir}}{\sim1}$ (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-fit Einasto shape parameter, $\alpha$, depends on peak height, $\nu$, in a manner that is accurately described by $\alpha {=}0.0070\nu^2{+}0.1839$. The distribution of the spin parameter, $\lambda$, has a weak dependence on equilibrium state; $\lambda$ peaks at roughly ${\sim}0.033$ for our relaxed sample, and at ${\sim}0.04$ for the full population. The spin--virial mass relation has a mild negative correlation at high redshift.