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Rapid evolution of the most luminous galaxies during the first 900 million years


The first 900 million years (Myr) to redshift z ≈ 6 (the first seven per cent of the age of the Universe) remains largely unexplored for the formation of galaxies. Large samples of galaxies have been found at z ≈ 6 (refs 1–4) but detections at earlier times are uncertain and unreliable. It is not at all clear how galaxies built up from the first stars when the Universe was about 300 Myr old (z ≈ 12–15) to z ≈ 6, just 600 Myr later. Here we report the results of a search for galaxies at z ≈ 7–8, about 700 Myr after the Big Bang, using the deepest near-infrared and optical images ever taken. Under conservative selection criteria we find only one candidate galaxy at z ≈ 7–8, where ten would be expected if there were no evolution in the galaxy population between z ≈ 7–8 and z ≈ 6. Using less conservative criteria, there are four candidates, where 17 would be expected with no evolution. This demonstrates that very luminous galaxies are quite rare 700 Myr after the Big Bang. The simplest explanation is that the Universe is just too young to have built up many luminous galaxies at z ≈ 7–8 by the hierarchical merging of small galaxies.

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Figure 1: Optical and near-infrared images of four candidate galaxies at z ≈ 7–8.
Figure 2: UV luminosity functions at redshift z ≈ 3, 6 and 7.4.
Figure 3: The luminosity density and star-formation rate density of the Universe over time.


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We are grateful to L. Bergeron, S. Kassin, D. Magee, M. Stiavelli, R. Thompson and A. Zirm for their help in reducing the NICMOS data, to M. Franx for critical reading of this manuscript, to S. Malhotra for making the NICMOS parallels to her PEARS program public early, and to I. Labbe for assistance in interpreting the IRAC data.

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Correspondence to Rychard J. Bouwens.

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Bouwens, R., Illingworth, G. Rapid evolution of the most luminous galaxies during the first 900 million years. Nature 443, 189–192 (2006).

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