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Early-type galaxies are considered to be the end-products of massive galaxyformation. Optical spectroscopic studies reveal that massive early-typegalaxies formed the bulk of their stars over short timescales (1<Gyr) and athigh redshift (z>2), followed by passive evolution to the present. However,their optical spectra are insensitive to constrain small episodes of recentstar formation, since they are dominated by old stars. Fortunately, thisproblem can be tackled in the ultraviolet range. While recent studies that makeuse of ultraviolet absorption lines have suggested the presence of young starsin a few early-type galaxies, the age and mass fractions of young stars andtheir dependence on galaxy mass, is unknown. Here we report a detailed study ofthese young stellar populations, from high-quality stacked spectra of 28,663galaxies from the BOSS survey, analysing optical and ultraviolet absorptionlines simultaneously. We find that residual star formation is ubiquitous inmassive early-type galaxies, measuring average mass fractions of ~0.5% in youngstars in the last 2 Gyr of their evolution. This fraction shows a decreasingtrend with galaxy stellar mass, consistent with a down-sizing scenario. We alsofind that synthetic galaxies from state-of-the-art cosmological numericalsimulations significantly overproduce both intermediate and young stellarpopulations. Therefore, our results pose stringent constraints on numericalsimulations of galaxy formation.

Sub one percent mass fractions of young stars in red massive galaxies