Aging halos: implications of secondary properties of dark matter halos on conditional statistics of stellar content of massive halos

Cold dark matter model predicts that the large-scale structure grows hierarchically. Small dark matter halos form first. Then, they grow gradually via continuous merger and accretion. These halos host the majority of baryonic matter in the Universe in the form of hot gas and cold stellar phase. Determining how baryons are partitioned into these phases requires detailed modeling of galaxy formation and their assembly history. It is speculated that formation time and accretion history of the same mass halos is correlated with their stellar content of the central galaxy. This implies that secondary properties of dark matter halo are responsible for the scatter about stellar-mass — halo mass relation. To evaluate this hypothesis, we employ halos of mass above 10¹⁴ M_sun realized by TNG300 solution of the IllustrisTNG project. We compute the conditional statistics of the stellar content of dark matter halos conditioned on their total mass and secondary properties. We find a strong correlation between formation time proxies and the stellar mass of central galaxies, but not the total stellar mass of halo.This finding implies that conditioning cluster observables on the magnitude gap, as a proxy of the formation time, can reduce the scatter about halo property–halo mass relation. Reduction in the scatter can be as significant as 30%, which implies more accurate halo mass prediction. Incorporating this information into mass—observable relation has the potential to improve cosmological constraints using halo abundance and allows us to gain insight into the baryon evolution within these systems.