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The radius of a dark matter halo is a fundamental quantity whose physical definition has largely remained ambiguous despite its application in almost every aspect of cosmology, largescale structure and galaxy formation. The classical virial and splashback radii are theoretical boundaries that describe the inner and outer regions of haloes, while the recently popular splashback radius gives insight into how matter accretes into haloes. In this talk I will present a simple yet physical and powerful new definition of the halo boundary, which is naturally defined at the minimum bias and maximum infall location around the halo. This boundary is created by the dynamic matter accretion process, which depletes matter from outside the boundary into the halo region within the boundary. As a result, this depletion radius clearly separates a growing and splashback-bounded halo from its draining environment. This is manifested in the spatial distribution as a depletion trough in the bias profile, where the correlation with the halo is the weakest relative to the correlation around a random particle, and can serve as the ideal radius for halo models of the largescale structure.
Dr. Matthew Fong is from SJTU.