The formation and evolution of massive compact galaxies in cosmological simulations

The population of quiescent galaxies undergoes significant changes over time. Ten billion years ago, these objects were generally compact and had a prominent disk with significant rotational support. In the local universe, however, they are larger by a factor of 3-5 and are mostly dispersion-supported ellipsoids. Still, there exists a rare population of galaxies that are still compact in the present-day universe, whose nature is still not well understood. One possibility is that these objects are galaxies formed at high redshifts that evolved passively. Or, they could have been formed at lower redshifts, but in similar conditions as the high redshift compact galaxies. In a recent work, our group studied a sample of compact quiescent galaxies from the MaNGA survey and found that, compared to average-sized quiescent galaxies, compact galaxies have similar ages, although being more metal rich and having a higher abundance of \(\alpha\) elements. However, while compact galaxies have a quiet accretion history, the opposite is true for average-sized galaxies. Thus, it is not clear whether the differences between the two populations are related to their high redshift progenitors or to a difference in their post-quenching evolution, e.g. the occurrence of mergers. Also, the local compact galaxies were shown to be the likely descendants of  post-starburst galaxies, which can play a role in the observed properties of these objects. Since it is not possible to recover a detailed star formation history from archeological studies of local compact quiescent galaxies, an alternative is to study cosmological simulations, which reproduce various observables and allow a thorough analysis of the evolution of galaxies across time. The aim of the work is to study the population of compact quiescent galaxies at \(z=0\) in the IllustrisTNG cosmological simulation and see how the evolution of these objects differ from that of the population of average-sized quiescent galaxies. We compare their kinematics, stellar populations, environmental features and star-formation histories in order to fully characterize the differences between them. These results are then compared to those of recent studies in the literature in order to propose a scenario for the evolution of compact quiescent galaxies.