How Young are Early-type Cluster Galaxies? Quantifying the Young Stellar Component in a Rich Cluster at [FORMULA][F]z=0.41[/F][/FORMULA]

Abstract

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 attention 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 () using Hubble Space Telescope/WFPC2 allow us to determine an NUV-optical color-magnitude relation, z p 0.41 whose slope is incompatible with a monolithic scenario for star formation at high redshift. A degeneracy between a young stellar component and its fractional mass contribution to the galaxy is found, and a photometric analysis comparing the data with the predictions for a simple two-stage star formation history is presented. The analysis shows that some of the early-type galaxies may have fractions higher than 10% of the total mass content in stars formed at . An increased scatter is found in the color-magnitude relation at the faint end, resulting in a z ∼ 0.5 significant fraction of faint blue early-type systems. This would imply that less massive galaxies undergo more recent episodes of star formation, and this can be explained in terms of a positive correlation between star formation efficiency and luminosity.