THE SPLASHBACK RADIUS AS A PHYSICAL HALO BOUNDARY AND THE GROWTH OF HALO MASS

Abstract

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 halo boundary choice is the radius at which accreted matter reaches its first orbital apocenter after turnaround. This splashback radius , , 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 and the enclosed mass, , as a function of mass accretion rate and peak height. We find that is in the range for rapidly accreting halos and is for slowly accreting halos. Thus, halos and their environmental effects can extend well beyond the conventionally defined "virial" radius. We show that and evolve relatively strongly compared to other commonly used definitions. In particular, evolves significantly even for the smallest dwarf-sized halos at z = 0. We also contrast with the mass enclosed within four scale radii of the halo density profile, , which characterizes the inner halo. During the early stages of halo assembly, and evolve similarly, but in the late stages stops increasing while 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.