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 ∼ 0.4. Rest-frame far UV emission is detected from a large fraction of so-called “passive spiral galaxies” –a significant population which exhibits spiral morphology with little or no spectroscopic evidence for 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/S0s 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 build-up of cluster S0s observed since z ≃0.4. Subject headings: galaxies: clusters: individual (Cl 0024+1654) — galaxies: spiral — galaxies: evolution — galaxies: stellar content — ultraviolet: galaxies