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.
