Luminosity-Dependent Assembly Bias of Central Galaxies from Weak Lensing and Clustering

Assembly bias, which is the variation in halo clustering at fixed mass driven by formation history, has long been predicted by numerical simulations but remains difficult to confirm observationally. Previous studies have reported evidence for halo assembly bias by dividing samples according to galaxy stellar mass using various methods. In this work, we present observational measurements of halo assembly bias based on the luminosity of spectroscopically confirmed brightest cluster galaxies (BCGs). Using cluster catalogs and shear measurements from the DESI Legacy Imaging Surveys, we employ a mass-dependent halo-bias model to disentangle halo bias from its underlying mass dependence in galaxy-galaxy lensing and clustering measurements. We confirm that brighter BCGs reside in more concentrated halos yet are less strongly clustered on large scales, with a relative bias ratio deviating from unity at the ~ 3σ level, providing clear evidence of assembly bias. The results are further supported by the FLAMINGO and MillenniumTNG hydrodynamical simulations, in which we directly detect assembly bias signals consistent with the observations, thereby strengthening the connection between galaxy luminosity and halo formation history.