Ultraviolet imaging of the galaxy cluster CL 0939+4713 (Abell 851) at z=0.41

The first UV F300W and F218W WFPC2 observations of the rich galaxy cluster CL 0939+4713 at z = 0.41 are presented and discussed. UV/optical two-color and color-magnitude diagrams of the sources detected in the F300W waveband are constructed. Thanks to preexisting Hubble Space Telescope (HST) optical images of the same field, a morphological classification for the majority of these objects is also provided. Moreover, taking advantage of recent red shift surveys along the CL 0939+4713 line of sight, separate diagrams compa ring the properties of galaxies belonging to the cluster and to its close p rojected held are presented. Possible evolutionary effects in the UV from z similar to 0.4 to the present time are investigated by comparing the rest- frame (mid-UV/optical) colors of galaxies in CL 0939+4713 with balloon-born e data of the Coma Cluster, as well as by resorting to suitable galaxy evol ution models. Finally, current attempts to constrain the epoch of the UV-up turn onset in evolved populations by means of HST UV observations are discu ssed.