Puzzling Ultra-Diffuse Galaxy Evolution (PUDGE)

The recent discovery of the most extended ultra-diffuse galaxy (UDG), Nube, has raised yet another question about the validity of the cold dark matter (CDM) model. The studies using cosmological and zoom-in simulations, which assume CDM, failed to replicate galaxies with the structural properties of Nube. However, the simulation box or the examined population of UDGs may be too narrow to fully capture the range of effects that can lead to the formation of such extraordinary galaxies. In this work we present a case study of a Nube-like galaxy from TNG100, the most extended simulated UDG examined to date that closely mirrors the structural properties of the observed Nube galaxy. Since its formation, the simulated Nube-like galaxy has already been ultra-diffuse and evolved mainly in isolated regions with occasional interactions. Its last major merger was finalized about 1.336 Gyr ago and left no trace of interaction apart from further extending the stellar size. This evolutionary pathway, featuring a recent merger that expanded an already ultra-diffuse stellar system, is unique and innovative compared to previous studies. We argue that multiple proposed formation mechanisms can operate simultaneously, further expanding the UDGs and making them extreme outliers of the mass-size relation under favorable conditions. Therefore, it is essential to study these simulated extreme outliers, their formation, and, more importantly, their evolution. We also highlight the necessity of carefully analyzing and interpreting the simulated data and better understanding the limitations of a chosen simulation. Thus, if Nube is considered an extreme outlier, its properties are not in tension with the standard cosmological model.