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Newcomers and suburbanites can drive the evolution of the size-stellar mass relation of early type galaxies in galaxy clusters

At fixed stellar mass $M_*$, the effective radius $R_{\rm e}$ of massive satellite early-type galaxies (ETGs) in galaxy clusters is, on average, larger at lower redshift. We study theoretically this size evolution using the state-of-the-art cosmological simulation IllustrisTNG100: we sampled $75$ simulated satellite ETGs at redshift $z=0$ with $M_* \ge 10^{10.4} M_{\odot}$ belonging to the two most massive ($\approx 10^{14.6} M_{\odot} $) haloes of the simulation. We traced back in time the two clusters' main progenitors and we selected their satellite ETGs at $z>0$ with the same criterion adopted at $z=0$. The $R_{\rm e}-M_*$ relation of the simulated cluster satellite ETGs, which is robustly measured out to $z=0.85$, evolves similarly to the observed relation over the redshift range $0\lesssim z \lesssim 0.85$. In the simulation the main drivers of this evolution are the acquisition of new galaxies ("newcomers") by the clusters and the transformation of member galaxies located at large clustercentric distance ("suburbanites") at $z=0.85$, which end up being massive satellite ETGs at $z=0$. Though several physical processes contribute to change the population of satellite ETGs in the considered redshift interval, the shape of the stellar mass function of the simulated cluster ETGs is not significantly different at $z=0.85$ and at $z=0$, consistent with observations.