Significant primordial star formation at redshifts z approximately 3-4

Four recent observational results have challenged our understanding of high-redshift galaxies, as they require the presence of far more ultraviolet photons than should be emitted by normal stellar populations. First, there is significant ultraviolet emission from Lyman break galaxies (LBGs) at wavelengths shorter than 912 A. Second, there is strong Lyman alpha emission from extended 'blobs' with little or no associated apparent ionizing continuum. Third, there is a population of galaxies with unusually strong Lyman alpha emission lines. And fourth, there is a strong He II (1,640 A) emission line in a composite of LBGs. The proposed explanations for the first three observations are internally inconsistent, and the fourth puzzle has remained hitherto unexplained. Here we show that all four problems are resolved simultaneously if 10-30 per cent of the stars in many galaxies at z approximately 3-4 are mainly primordial--unenriched by elements heavier than helium ('metals'). Most models of hierarchical galaxy formation assume efficient intragalactic metal mixing, and therefore do not predict metal-free star formation at redshifts significantly below z approximately 5. Our results imply that micromixing of metals within galaxies is inefficient on an approximately gigayear timescale, a conclusion that can be verified with higher-resolution simulations, and future observations of the He ii emission line.