Optically-passive spirals: The missing link in gradual star formation
  suppression upon cluster infall
  Galaxies migrate from the blue cloud to the red sequence when their star
formation is quenched. Here, we report on galaxies quenched by environmental
effects and not by mergers or strong AGN as often invoked: They form stars at a
reduced rate which is optically even less conspicuous, and manifest a
transition population of blue spirals evolving into S0 galaxies. These
'optically passive' or 'red spirals' are found in large numbers in the STAGES
project (and by Galaxy Zoo) in the infall region of clusters and groups.
