Where do galaxy clusters really end? A new boundary from light alone

The splashback radius marks where material stops orbiting a galaxy cluster and begins falling inward—a natural, physics-based edge independent of how astronomers define cluster size. Using SDSS imaging data and a probabilistic membership method, researchers measured splashback radii for 500 clusters and derived a mass–size scaling relation across redshifts. They then applied this relation to 15,000 photometric clusters to construct the first splashback mass function from images alone, finding good agreement with simulations at high masses and explaining lower-mass discrepancies by known catalog limits.