UV properties of early-type galaxies in the Virgo Cluster

We study the UV properties of a volume-limited sample of early-type galaxies in the Virgo Cluster combining new GALEX far-ultraviolet (1530 angstrom) and near-ultraviolet (2310 angstrom) data with spectrophotometric data available at other wavelengths. The sample includes 264 elliptical, lenticular, and dwarf galaxies spanning a large range in luminosity (M-B greater than or equal to - 15). While the NUV to optical or near-IR color-magnitude relations (CMRs) are similar to those observed at optical wavelengths, with a monotonic reddening of the color index with increasing luminosity, the (FUV - V) and (FUV - H) CMRs show a discontinuity between massive and dwarf objects. An even more pronounced dichotomy is observed in the (FUV - NUV) CMR. For elliptical galaxies, the (FUV - NUV) color becomes bluer with increasing luminosity and with increasing reddening of the optical or near-IR color indices. For the dwarfs, the opposite trend is observed. These observational evidences are consistent with the idea that the UV emission is dominated by hot, evolved stars in giant systems, while in dwarf ellipticals residual star formation activity is more common.