The Non-universal Pseudo Phase-Space Density Profiles of Symphony Host Halos

Abstract

Cosmological N-body simulations have long suggested that the pseudo phase-space density (PPSD), $\rho/\sigma^3$, of cold dark matter haloes follows the universal relation $\rho/\sigma^3 \propto r^{\chi}$, with $\chi \approx -1.875$, as predicted by spherical secondary-infall similarity solutions. This power law appears to hold despite the fact that neither the density $\rho(r)$ nor velocity dispersion $\sigma(r)$ follow universal power law relations individually, even at fixed mass. We analyze 246 host haloes from the \textit{Symphony} suite of high-resolution cosmological zoom-in simulations, to consistently measure PPSD profiles across host masses from $10^{11}$ to $10^{15} M_\odot$. We find that the PPSD systematically deviates from a power law, and that haloes with larger deviations from Jeans equilibrium systematically develop steeper average PPSD slopes. This result suggests that the PPSD is not universal; instead, it is linked to a halo's degree of dynamical equilibrium, which is ultimately set by halo formation history. As a result, we show that secondary halo properties such as concentration and accretion rate inherit significant correlations with the PPSD slope. Moreover, our hosts' PPSD profiles are remarkably consistent with predictions from one-dimensional self-similar fluid collapse models, indicating that three-dimensional structure, velocity anisotropy, and filamentary accretion all play negligible roles in shaping the PPSD. Thus, we argue that the PPSD is shaped by mass assembly alone, and that its non-universality reflects the diversity of halo growth histories.