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Suppression of dwarf galaxy formation by cosmic reionization


A large number of faint galaxies, born less than a billion years after the Big Bang, have recently been discovered1,2,3,4,5,6. Fluctuations in the distribution of these galaxies contributed to a scatter in the ionization fraction of cosmic hydrogen on scales of tens of megaparsecs, as observed along the lines of sight to the earliest known quasars7,8,9. Theoretical simulations predict that the formation of dwarf galaxies should have been suppressed after cosmic hydrogen was reionized10,11,12,13, leading to a drop in the cosmic star-formation rate14. Here we report evidence for this suppression. We show that the post-reionization galaxies that produced most of the ionizing radiation at a redshift z ≈ 5.5 must have had a mass in excess of 1010.9 ± 0.5 solar masses (M) or else the aforementioned scatter would have been smaller than observed. This limiting mass is two orders of magnitude larger than the galaxy mass that is thought to have dominated the reionization of cosmic hydrogen (108M). We predict that future surveys with space-based infrared telescopes will detect a population of smaller galaxies that reionized the Universe at an earlier time, before the epoch of dwarf galaxy suppression.

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Figure 1: Constraints on the star-formation efficiency ( fstar), duty cycle ( tlt/tH) and minimum virial temperature ( Tmin) of galaxies at z ≈ 5.5–6.


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This work was supported in part by grants from ARC, NSF and NASA. We acknowledge helpful discussions with A. Lidz regarding this work.

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Correspondence to J. Stuart B. Wyithe or Abraham Loeb.

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Reprints and permissions information is available at The authors declare no competing financial interests.

Supplementary information

Supplementary Discussion

The Letter to Nature with the above title interprets the observed luminosity function of z~6 galaxies and the large-scale fluctuations in the Lyman-alpha optical depth towards z~6 quasars, based on a detailed theoretical model. This supplement provides full details on the basic version of this model as well as its variants. (PDF 620 kb)

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Wyithe, J., Loeb, A. Suppression of dwarf galaxy formation by cosmic reionization. Nature 441, 322–324 (2006).

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