Journal
ASTROPHYSICAL JOURNAL
Volume 541, Issue 2, Pages L37-L40Publisher
UNIV CHICAGO PRESS
DOI: 10.1086/312898
Keywords
galaxies : clusters : individual (Abell 851, Cl 0939+4713); galaxies : elliptical and lenticular, cD; galaxies : evolution; galaxies : formation
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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 (z = 0.41) using Hubble Space Telescope/WFPC2 allow us to determine an NW-optical color-magnitude relation, 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 z similar to 0.5. An increased scatter is found in the color-magnitude relation at the faint end, resulting in a 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.
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