Inner structure of cold and warm dark matter halos from particle dynamics

Abstract

Using the number of apocenter passages $p$ and the radial action $J_r$ of each particle, we characterize the phase-space structure within the multi-stream regions of cold and warm dark matter halos in cosmological $N$-body simulations. Building on previous work by Enomoto et al. (2024), we analyze the radial density profiles of particles classified by $p$ and $J_r$. We find that the profiles consistently follow a double power-law structure, independent of the dark matter model or halo mass. The inner profile exhibits a $\rho \propto r^{-1}$ behavior, which is consistent with previous studies. Notably, this characteristics persist across both classification schemes. In contrast, the outer power-law profiles display distinct behaviors depending on the classification. While particles classified by $p$ exhibit a steeper slope, ranging from $-6$ to $-8$, those classified by $J_r$ follow a common slope of approximately $-3.5$. Overall, the amplitude of the double power-law profiles varies between simulations for different dark matter models, but this variation can be attributed to statistical differences in the concentration of halos across the models.