The Connection between Halo Assembly History and the Stellar Distribution I: The Central Stellar Mass - Richness Plane

Recent observations suggest intrinsic links between the stellar components of massive halos and their assembly histories. Using massive halos with [M halo/M] at least 13.0 from the IllustrisTNG-300 simulation, we explore differences in secondary halo properties between samples selected by central galaxy stellar mass and satellite richness. For group-like masses, we find that halos selected by the outskirts stellar mass of their central galaxies between 50 and 100 kpc (M,[50,100]) have similar halo mass distributions to those selected by intrinsic satellite richness (λ 10, R 200), but differ significantly in assembly history: M,[50,100] -selected halos are more concentrated and formed earlier. At higher masses (about10 14 M), intrinsic richness becomes the better halo-mass proxy, but only when projection effects are ignored. We find that the difference-set test selects comparable halo-mass samples most effectively when the two proxies have similar scatter. Projection effects and baryonic physics do not alter the main trends but remain important caveats for observational applications. Our findings show that the central stellar mass-richness plane is a powerful tool for selecting halos with particular accretion histories, offering new insights into the galaxy-halo connection in the era of deep imaging and precision lensing surveys such as Euclid and LSST.