Ultraviolet Imaging of the Galaxy Cluster CL 0939+4713 (Abell 851) at z=0.41*

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Abstract
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 redshift surveys along the CL 0939+4713 line of sight, separate diagrams comparing the properties of galaxies belonging to the cluster and to its close projected field are presented. Possible evolutionary effects in the UV from z ~ 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-borne data of the Coma Cluster, as well as by resorting to suitable galaxy evolution models. Finally, current attempts to constrain the epoch of the UV-upturn onset in evolved populations by means of HST UV observations are discussed.
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