Splashback Shells of Cold Dark Matter Halos

Abstract

The density field in the outskirts of dark matter halos is discontinuous due to a caustic formed by matter at its first apocenter after infall. In this paper, we present an algorithm to identify the "splashback shell" formed by such apocenters in individual simulated halos using only a single snapshot of the density field. We implement this algorithm in the code SHELLFISH (SHELL Finding In Spheroidal Halos) and present a series of tests demonstrating that the code identifies splashback shells correctly and measures their properties with an accuracy of \(< 5\%\) for halos with more than 50,000 particles and mass accretion rates of \(\Gamma > 0.5\). Using this code, we present the first estimates for several basic properties of splashback shells, such as \(R_{\rm sp}\), shape, and orientation and their dependence on redshift and \(\Gamma\). We find that splashback shells generally have non-ellipsoidal oval shapes. We confirm previous findings that \(R_{\rm sp}\) decreases with increasing \(\Gamma\), but we show that it also depends strongly on the peak height, \(\nu\). Halos with larger peak heights have systematically smaller values of \(R_{\rm sp}/R_{\rm 200m}\) at a fixed accretion rate. We provide accurate fits for the distribution of \(R_{\rm sp}\) and the density contrasts enclosed within splashback shells as a function of \(\Gamma\), \(\nu\), and \(z.\) We find that the splashback radii estimated by SHELLFISH for halos with high accretion rates are \(20\%-30\%\) larger than those estimated by previous studies through the use of stacked density profiles. We demonstrate that the latter are biased low due to the contributions of high-mass subhalos to these profiles. We show that using the median instead of mean density in each radial bin mitigates the effect of substructure on density profiles, removing the bias and bringing profile-based estimates of \(R_{\rm sp}\) into agreement with the results of SHELLFISH.