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The signature of the first stars in atomic hydrogen at redshift 20


Dark and baryonic matter moved at different velocities in the early Universe, which strongly suppressed star formation in some regions1. This was estimated2 to imprint a large-scale fluctuation signal of about two millikelvin in the 21-centimetre spectral line of atomic hydrogen associated with stars at a redshift of 20, although this estimate ignored the critical contribution of gas heating due to X-rays3,4 and major enhancements of the suppression. A large velocity difference reduces the abundance of haloes1,5,6 and requires the first stars to form in haloes of about a million solar masses7,8, substantially greater than previously expected9,10. Here we report a simulation of the distribution of the first stars at redshift 20 (cosmic age of around 180 million years), incorporating all these ingredients within a 400-megaparsec box. We find that the 21-centimetre hydrogen signature of these stars is an enhanced (ten millikelvin) fluctuation signal on the hundred-megaparsec scale, characterized2 by a flat power spectrum with prominent baryon acoustic oscillations. The required sensitivity to see this signal is achievable with an integration time of a thousand hours with an instrument like the Murchison Wide-field Array11 or the Low Frequency Array12 but designed to operate in the range of 50–100 megahertz.

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Figure 1: The effect of relative velocity on the distribution of star-forming haloes at z = 20.
Figure 2: The effect of relative velocity on the distribution of star-forming haloes at z = 40.
Figure 3: The effect of relative velocity on the 21-cm brightness temperature at z = 20.
Figure 4: The signature of relative velocity in the 21-cm power spectrum at z = 20.


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This work was supported by the Israel Science Foundation (for R.B., and E.V.’s stay at Tel Aviv University) and by the European Research Council (for A.F.). D.T. and C.M.H. were supported by the US Department of Energy and the National Science Foundation. C.M.H. is also supported by the David & Lucile Packard Foundation.

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Authors and Affiliations



R.B. initiated the project, and E.V. made the computations and figures by developing a code, parts of which were based on codes supplied by A.F., D.T. and C.M.H. The text was written by R.B. and edited by the other authors. A.F. added the L–W module for Supplementary section 3 and made Supplementary Fig. 1.

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Correspondence to Eli Visbal or Rennan Barkana.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Text and Data 1-4, which comprises 1) a description of the simulation code; 2) a comparison with previous work on the dark matter to baryon velocity difference; 3) the expected timing of the three high-redshift feedback transitions and 4) observational considerations. Supplementary Figure 1 (on the Lyman-Werner feedback) and additional references are also included. (PDF 252 kb)

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Visbal, E., Barkana, R., Fialkov, A. et al. The signature of the first stars in atomic hydrogen at redshift 20. Nature 487, 70–73 (2012).

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