How young are early-type cluster galaxies? Quantifying the young stellar component in a rich cluster at z=0.41

We present a new method of quantifying the mass fraction of young stars in galaxies by analyzing near-ultraviolet (NUV)-optical colors. We focus our a ttention on early-type cluster galaxies, whose star formation history is at present undetermined. Rest-frame NUV (F300W) and optical (F702W) images of cluster Abell 851 (z = 0.41) using Hubble Space Telescope/WFPC2 allow us t o determine an NW-optical color-magnitude relation, whose slope is incompat ible with a monolithic scenario for star formation at high redshift. A dege neracy between a young stellar component and its fractional mass contributi on to the galaxy is found, and a photometric analysis comparing the data wi th the predictions for a simple two-stage star formation history is present ed. The analysis shows that some of the early-type galaxies may have fracti ons higher than 10% of the total mass content in stars formed at z similar to 0.5. An increased scatter is found in the color-magnitude relation at th e faint end, resulting in a significant fraction of faint blue early-type s ystems. This would imply that less massive galaxies undergo more recent epi sodes of star formation, and this can be explained in terms of a positive c orrelation between star formation efficiency and luminosity.