The Splashback Radius as a Physical Halo Boundary and the Growth of Halo Mass

The boundaries of cold dark matter halos are commonly defined to enclose a density contrast Δ relative to a reference (mean or critical) density. We argue that a more physical boundary of halos is the radius at which accreted matter reaches its first orbital apocenter after turnaround. This splashback radius, Rₛₚ, manifests itself as a sharp density drop in the halo outskirts, at a location that depends upon the mass accretion rate. We present calibrations of Rₛₚ and the enclosed mass, Mₛₚ, as a function of the accretion rate and alternatively peak height. We find that Rₛₚ varies between ≈0.8-1R₂₀₀ₘ for rapidly accreting halos and ≈1.5R₂₀₀ₘ for slowly accreting halos. The extent of a halo and its associated environmental effects can thus extend well beyond the conventionally defined "virial" radius. We show that Mₛₚ and Rₛₚ evolve relatively strongly compared to other commonly used definitions. In particular, Mₛₚ evolves significantly even for the smallest dwarf-sized halos at $z=0$. We also contrast Mₛₚ with the mass enclosed within four scale radii of the halo density profile, M<4rs, which characterizes the inner halo. During the early stages of halo assembly, Mₛₚ and M<4rs evolve similarly, but in the late stages M<4rs stops increasing while Mₛₚ continues to grow significantly. This illustrates that halos at low z can have "quiet" interiors while continuing to accrete mass in their outskirts. We discuss potential observational estimates of the splashback radius and show that it may already have been detected in galaxy clusters.