1. Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, Pennsylvania 19104, USA
2. Department of Astronomy, Columbia University, 550 West 120th Street, New York, New York 10027, USA

Correspondence to: Raul Jimenez¹Zoltan Haiman² Correspondence and requests for materials should be addressed to R.J. (Email: raulj@physics.upenn.edu) or Z.H. (Email: zoltan@astro.columbia.edu).

Abstract

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 emission¹ from Lyman break galaxies (LBGs) at wavelengths shorter than 912 Å. Second, there is strong Lyman emission² from extended 'blobs' with little or no associated apparent ionizing continuum. Third, there is a population of galaxies with unusually strong Lyman emission lines³. And fourth, there is a strong He ii (1,640 Å) emission line⁴ 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 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^{5, 6, 7, 8, 9} metal-free star formation at redshifts significantly below z 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.

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