The age gradients of galaxies in EAGLE: outside-in quenching as the
  origin of young bulges in cluster galaxies
  Many disc galaxies in clusters have been found with bulges of similar age or
younger than their surrounding discs, at odds with field galaxies of similar
morphology and their expected inside-out formation. We use the EAGLE
simulations to test potential origins for this difference in field and cluster
galaxies. We find, in agreement with observations, that on average
disc-dominated field galaxies in the simulations have older inner regions,
while similar galaxies in groups and clusters have similarly aged or younger
inner regions. This environmental difference is a result of outside-in
quenching of the cluster galaxies. Prior to group/cluster infall, galaxies of a
given present-day mass and morphology exhibit a similar evolution in their
specific star formation rate (sSFR) profiles. Post-infall, the outer sSFRs of
group and cluster galaxies significantly decrease due to interstellar medium
stripping, while the central sSFR remains similar to field galaxies. Field disc
galaxies instead generally retain radially increasing sSFR profiles. Thus,
field galaxies continue to develop negative age gradients (younger discs),
while cluster galaxies instead develop positive age gradients (younger bulges).
