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Extremely metal-poor gas at a redshift of 7


In typical astrophysical environments, the abundance of heavy elements ranges from 0.001 to 2 times the solar value. Lower abundances have been seen in selected stars in the Milky Way’s halo1,2,3 and in two quasar absorption systems at redshift z = 3 (ref. 4). These are widely interpreted as relics from the early Universe, when all gas possessed a primordial chemistry. Before now there have been no direct abundance measurements from the first billion years after the Big Bang, when the earliest stars began synthesizing elements. Here we report observations of hydrogen and heavy-element absorption in a spectrum of a quasar at z =  7.04, when the Universe was just 772 million years old (5.6 per cent of its present age). We detect a large column of neutral hydrogen but no corresponding metals (defined as elements heavier than helium), limiting the chemical abundance to less than 1/10,000 times the solar level if the gas is in a gravitationally bound proto-galaxy, or to less than 1/1,000 times the solar value if it is diffuse and unbound. If the absorption is truly intergalactic5,6, it would imply that the Universe was neither ionized by starlight nor chemically enriched in this neighbourhood at z ≈ 7. If it is gravitationally bound, the inferred abundance is too low to promote efficient cooling7,8, and the system would be a viable site to form the predicted but as yet unobserved massive population III stars.

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Figure 1: Infrared spectrum of ULAS J1120+0641, compared to our estimate of the intrinsic quasar spectrum without foreground absorption.
Figure 2: Continuum-normalized transmitted flux in spectral regions where expected heavy-element transitions would appear for a DLA at redshift z = 7.041.
Figure 3: Comparison of our z = 7.04 abundance measurement in ULAS J1120 with existing values from the literature.

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We thank J. O’Meara and A. Frebel for comments during the preparation of this Letter. M. Haehnelt also provided advice on methods for modelling the quasar near-zone, and G. Richards shared his composite QSO spectra in electronic form. This work includes data gathered with the 6.5-m Magellan Telescopes located at Las Campanas Observatory, Chile. R.A.S. acknowledges support from the NSF under awards AST-0908920 and AST-1109115. K.L.C. is supported by the NSF Astronomy and Astrophysics Postdoctoral Fellowship programme.

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Authors and Affiliations



R.A.S. constructed the FIRE instrument, and together with P.W.S. designed and executed the observations, performed the analysis and prepared the manuscript. K.L.C. prepared observations and edited the manuscript. M.S.M. assisted with the pipeline software used to reduce the spectroscopic data, and M.M.K. wrote the software to perform eigenspectrum continuum fits. A.J.B. contributed to the spectrograph construction, and executed observations for the program. All authors helped with the scientific interpretations and commented on the manuscript.

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Correspondence to Robert A. Simcoe.

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The authors declare no competing financial interests.

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Simcoe, R., Sullivan, P., Cooksey, K. et al. Extremely metal-poor gas at a redshift of 7. Nature 492, 79–82 (2012).

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