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Significant primordial star formation at redshifts z ≈ 3–4

An Erratum to this article was published on 04 May 2006


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 α emission2 from extended ‘blobs’ with little or no associated apparent ionizing continuum. Third, there is a population of galaxies with unusually strong Lyman α 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 predict5,6,7,8,9 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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Figure 1: Lyman break galaxies stacked observed spectrum and best-fit population synthesis models.
Figure 2: Colour distributions of the Ly α blobs.

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We thank Y. Matsuda and T. Yamada for providing the unpublished raw colour data that appears in Fig. 2, D. Schaerer for providing pop-III spectra in electronic form, and A. Shapley for useful discussions. We also thank T. Beers for pointing out the C trends in low metallicity stars and suggesting that these could be explained by our model. R.J. and Z.H. gratefully acknowledge financial support from NSF and NASA.

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Correspondence to Raul Jimenez or Zoltan Haiman.

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Supplementary Discussion

Additional discussion on modeling the 912AA break from the spectra of Lyman break galaxies. (PDF 29 kb)

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Jimenez, R., Haiman, Z. Significant primordial star formation at redshifts z ≈ 3–4. Nature 440, 501–504 (2006).

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