Multistream radial structure of cold dark matter haloes from particle trajectories: deep inside splashback radius

Abstract

By tracking trajectories of dark matter (DM) particles accreting on to haloes in cosmological N-body simulations, we investigate the radial phase-space distribution of cold dark matter (CDM) haloes, paying attention to their inner regions deep inside the halo boundary called the splashback radius, where the particles undergo multistream flows. Improving the analysis by Sugiura et al., we classify DM particles by the number of apocentre passages, p, and count it up to p = 40 for each halo over a wide mass range. Quantifying the radial density profile for particles having the same value of p, we find that it generally exhibits a double power-law feature, whose indices of inner and outer slopes are well described by −1 and −8, respectively. Its characteristic scale and density are given as a simple fitting function of p, with a weak halo mass dependence. Interestingly, summing up these double power-law profiles beyond p = 40 reproduces well the total density profile of simulated haloes. The double power-law nature is persistent and generic not only in mass-selected haloes but also in haloes selected in different criteria. Our results are compared with self-similar solutions that describe the stationary and spherical accretion of DM. We find that even when introducing a non-zero angular momentum, none of them explain the radial multistream structure. The analysis with particle trajectories tracing back to higher redshifts suggests that the double power-law nature has been established during an early accretion phase and remains stable.