a) Radial density profile of model galaxy 1nb after 2 Gyr of evolution in the vacuum. We show two gas phases used in the simulation, named hot (ionized) and cold gas. In black, the old stellar disc is plotted and we show in yellow the newly formed stars within the 2 Gyr of evolution. Owing to the ring-like structure and the method of calculating the profile, it seems that some gas is missing at a radius of 15, 20, and 25 kpc. b) Face-on view of the galaxy’s gas component. Already after 2 Gyr, overdensities in concentric rings are formed. Furthermore, a spiral structure can be seen.