A simulation view on the formation of ultra-diffuse galaxies in the field and in galaxy groups

Ultra-diffuse galaxies (UDGs) with dwarf stellar masses and Milky Way sizes appear to be ubiquitous in groups and clusters and are also observed in the field. We study such galaxies in cosmological zoom-in simulations, aiming at understanding their formation both in groups and in the field. We find that while field UDGs arise from dwarfs in a specific mass range from successive episodes of supernova feedback, group UDGs can also form by tidal puffing up and become quiescent by ram-pressure stripping. The host haloes of both field and group UDGs have typical spin but significant cores. Field UDGs tend to be dark-matter dominated towards their center and to be more prolate than dwarf galaxies of similar mass. In groups, satellite dwarfs can become UDGs after pericenter passage by tidal heating and simultaneously lose most of their gas by ram-pressure stripping, suppressing star formation and inducing a color gradient in agreement with observations.