The size and stellar mass evolution of satellite ETGs in the most massive clusters

In this thesis work we analyse the evolution of the size-stellar mass relation of early-type galaxies (ETGs) in clusters of galaxies. Starting from the state-of-the-art cosmological simulation IllustrisTNG100-1, 90 ETGs aresampled at z=0 with a total stellar mass (provided on the fly by the simulation itself) greater than 10^10.4 M and belonging to the two most massive clusters of the simulation at that redshift (M200≈10^14.6Mfor both). The growth curves of the sample are measured to derive four different estimates of the projected stellar half-mass radius from four definitions of total stellar mass. With these radii and masses, the best fits of the size-stellar mass relations are constructed and they are compared with COMA observations (z= 0.02): the best agreement occurs with the definition of total stellar mass given by the stellar mass within twice the 3D stellar half-mass radius, therefore we use that definition to compare simulations to observations to all z≤1.8. Subsequently, the two clusters’ main progenitors are followed backwards in time as z increases and new ETGs are selected with the same procedure seen at z= 0. The relations found for the simulated sample are reliable out to z=0.85 and they are qualitatively in agreement with the observations: as z increases, ETGs with the same stellar mass tend to be more compact. Finally, we find that the main mechanism that modifies the size-stellar mass relation over cosmic time is the acquisition of new ETGs (most of them star forming at z= 0.85) by the two clusters between z=0.85 and z=0: these ETGs are more extended than their counterparts with the same stellar mass already present in the clusters. Moreover, we find that the stellar mass function of the simulated ETGs is not significantly different at z=0.85 and at z=0.