Properties of Cold Dark Matter Halos at z>6

Abstract

We compute the properties of dark matter halos with mass $10^{6.5}-10^9\msun$ at redshift $z=6-11$ in the standard cold dark matter cosmological model, utilizing a very high resolution N-body simulation. We find that dark matter halos in these mass and redshift ranges are significantly biased over matter with a bias factor in the range 2-6. The dark matter halo mass function displays a slope of $2.05\pm 0.15$ at the small mass end. We do not find a universal dark matter density profile. Instead, we find a significant dependence of the central density profile of dark matter halos on halo mass and epoch with $α_0=0.4-1.0$; the high-mass ($M\ge 10^8\msun$) low-redshift ($z\sim 6$) halos occupy the high end of the range and low-mass ($M\sim 10^{7}\msun$) high-redshift ($z\sim 11$) halos occupy the low end. Additionally, for fixed mass and epoch there is a significant dispersion in $α_0$ due to the stochastic assembly of halos. Our results fit a relationship of the form $α_0=0.75((1+z)/7.0)^{-1.25}(M/10^7\msun)^{0.11(1+z)/7.0}$ with a dispersion about this fit of $\pm 0.5$ and no systematic dependence of variance correlated with environment. The median spin parameter of dark matter halos is $0.03-0.04$ but with a large lognormal dispersion of $\sim 0.4$. Various quantities are tabulated or fitted with empirical formulae.

38 pages, submitted to ApJ, name revised (the first author name was wrong)