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This paper examines star formation (SF) in relatively massive, primarilyearly-type galaxies (ETGs) at z~0.1. A sample is drawn from bulge-dominatedGALEX/SDSS galaxies on the optical red sequence with strong UV excess and yetquiescent SDSS spectra. High-resolution far-UV imaging of 27 such ETGs usingHST ACS/SBC reveals structured UV morphology in 93% of the sample, consistentwith low-level ongoing SF (~0.5 Ms/yr). In 3/4 of the sample the SF is extendedon galaxy scales (25-75 kpc), while the rest contains smaller (5-15 kpc) SFpatches in the vicinity of an ETG - presumably gas-rich satellites beingdisrupted. Optical imaging reveals that all ETGs with galaxy-scale SF in oursample have old stellar disks (mostly S0 type). None is classified as a trueelliptical. In our sample, galaxy-scale SF takes the form of UV rings ofvarying sizes and morphologies. For the majority of such objects we concludethat the gas needed to fuel current SF has been accreted from the IGM, probablyin a prolonged, quasi-static manner, leading in some cases to additional diskbuildup. The remaining ETGs with galaxy-scale SF have UV and opticalmorphologies consistent with minor merger-driven SF or with the final stages ofSF in fading spirals. Our analysis excludes that all recent SF on the redsequence resulted from gas-rich mergers. We find further evidence thatgalaxy-scale SF is almost exclusively an S0 phenomenon (~20% S0s have SF) byexamining the overall optically red SDSS ETGs. Conclusion is that significantnumber of field S0s maintain or resume low-level SF because the preventivefeedback is not in place or is intermittent. True ellipticals, on the otherhand, stay entirely quiescent even in the field.

Galaxy-scale Star Formation on the Red Sequence: the Continued Growth of S0s and the Quiescence of Ellipticals