GALEX observations of passive spirals in the cluster Cl 0024+17:: Clues to the formation of S0 galaxies

We present new results from deep GALEX UV imaging of the cluster Cl 0024 + 17 at z similar to 0.4. Rest-frame far UV emission is detected from a large fraction of so- called passive spiral galaxies - a significant population that exhibits spiral morphology with little or no spectroscopic evidence of ongoing star formation. This population is thought to represent infalling galaxies whose star formation has been somehow truncated by environmental processes, possibly in morphological transition to S0 galaxies. Compared to normal cluster spirals, we find that passive spirals are redder in FUV-optical color, while exhibiting much stronger UV emission than cluster E/S0 galaxies - as expected for recently truncated star formation. By modeling the different temporal sensitivities of UV and spectroscopic data to recent activity, we show that star formation in passive spirals decayed on timescales of less than 1 Gyr, consistent with gas starvation - a process where the cluster environment prevents cold gas from accreting onto the spiral disk. Intriguingly, the fraction of spirals currently observed in the passive phase is consistent with the longer period expected for the morphological transformation and the subsequent buildup of cluster S0 galaxies observed since z similar or equal to 0.4.