Speaker
Description
Deep wide-field imaging (e.g., HSC, Euclid) now reveals the low-surface-brightness outskirts of massive central galaxies and intra-cluster light, while cluster spectroscopic surveys (e.g., JUST) can better probe their satellite populations. Together, these two stellar components preserve complementary records of the host halo’s assembly history. Using a halo-mass-complete sample of massive halos from the IllustrisTNG300 simulation, we develop a framework that reveals this record through these two complementary observables. On the central stellar mass-richness plane, we show that the outskirt stellar mass of the central galaxy (e.g., $M_{\star,[50,100] kpc}$) rivals satellite richness as a halo mass proxy while tracing a different stage of assembly. At fixed halo mass, halos with more prominent outskirts but fewer satellites formed earlier and are more concentrated, a trend that can be tested directly with stacked galaxy–galaxy lensing. Resolving the central galaxy radially, we further show that its stellar profile is a stratified fossil record of halo assembly. Since stellar at larger radii tended to be accreted at later times, the stellar mass at larger radii retains a memory of the halo mass at later cosmic epochs, with this memory peaking at z~1 in the central core and at z~0.7 in the outer stellar halo, so that a single profile measurement maps out the halo’s growth across cosmic time. We also find that the central stellar mass identifies the most massive halo progenitors at intermediate redshift more faithfully than satellite richness.
These results establish the BCG+ICL+Satellite as a practical, time-resolved probe of halo assembly history. In the era of multi-cluster-probes, spectroscopic membership and dynamical masses for thousands of clusters will suppress projection systematics and calibrate these stellar assembly tracers against weak lensing, clustering and other probes, opening a new window on halo assembly bias and cluster cosmology.