Ring Galaxies in the EAGLE Hydrodynamical Simulations

We study the formation and evolution of ring galaxies in the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulations. We use the largest reference model Ref-L100N1504, a cubic cosmological volume of 100 comoving megaparsecs on a side, to identify and characterise these systems through cosmic time. The number density of ring galaxies in EAGLE is in broad agreement with the observations. The vast majority of ring galaxies identified in EAGLE (83 per cent) have an interaction origin, i.e., form when one or more companion galaxies drop-through a disk galaxy. The remainder (17 per cent) have very long-lived ring morphologies (> 2 Gyr) and host strong bars. Ring galaxies are HI rich galaxies, yet display inefficient star formation activity and tend to reside in the green valley particularly at z > 0.5. This inefficiency is mainly due to the low pressure and metallicity of their interstellar medium (ISM) compared with the ISM of similar star-forming galaxies. We find that the interaction(s) is responsible for decreasing the ISM pressure by causing the ISM gas to flow from the inner regions to the outer disk, where the ring feature forms. At a fixed radius, the star formation efficiency of ring galaxies is indistinguishable from their star-forming counterparts, and thus the main reason for their integrated lower efficiency is the different gas surface density profiles. Since galaxy morphologies are not used to tune the parameters in hydrodynamical simulations, the experiment performed here demonstrates the success of the current numerical models in EAGLE.