The dependence of dark halo clustering on the formation epoch and the concentration parameter

We examine the age-dependence of dark matter halo clustering in an unprecedented accuracy using a set of 7 high-resolution cosmological simulations each with $N=1024^3$ particles. We measure the bias parameters for halos over a large mass range using the cross-power-spectrum method that can effectively suppress the random noise even in the sparse sampling of the most massive halos. This enables us to find, for the first time, that younger halos are more strongly clustered than older ones for halo masses $M>10M_{\ast}$, where $M_\ast$ is the characteristic nonlinear mass scale. For $M<M_{\ast}$, our results confirm the previous finding of Gao et al. that older halos are clustered more strongly than the younger ones. We also study the halo bias as a function of halo concentration, and find that the concentration dependence is weaker than the age dependence for $M<M_{\ast}$, but stronger for $M\ga 50 M_{\ast}$. The accurate and robust measurement of the age dependences of halo bias points to a limitation of the simple excursion set theory which predicts that the formation and structure of a halo of given mass is independent of its environment.