Searches for very-high-redshift galaxies over the past decade have yielded a large sample of more than 6,000 galaxies existing just 900–2,000 million years (Myr) after the Big Bang (redshifts 6 > z > 3; ref. 1). The Hubble Ultra Deep Field (HUDF09) data2,3 have yielded the first reliable detections of z ≈ 8 galaxies3,4,5,6,7,8,9 that, together with reports of a γ-ray burst at z ≈ 8.2 (refs 10, 11), constitute the earliest objects reliably reported to date. Observations of z ≈ 7–8 galaxies suggest substantial star formation at z > 9–10 (refs 12, 13). Here we use the full two-year HUDF09 data to conduct an ultra-deep search for z ≈ 10 galaxies in the heart of the reionization epoch, only 500 Myr after the Big Bang. Not only do we find one possible z ≈ 10 galaxy candidate, but we show that, regardless of source detections, the star formation rate density is much smaller (∼10%) at this time than it is just ∼200 Myr later at z ≈ 8. This demonstrates how rapid galaxy build-up was at z ≈ 10, as galaxies increased in both luminosity density and volume density from z ≈ 10 to z ≈ 8. The 100–200 Myr before z ≈ 10 is clearly a crucial phase in the assembly of the earliest galaxies.
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We are grateful to all those at NASA, STScI and throughout the community who have worked to make the Hubble Space Telescope the observatory that it is today, and we acknowledge the importance of the servicing missions and those who organised them. We acknowledge our program coordinator W. Januszewski for his care in helping to set up our program and observing configuration. We acknowledge support from NASA and the Swiss National Science Foundation.
The authors declare no competing financial interests.
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Bouwens, R., Illingworth, G., Labbe, I. et al. A candidate redshift z ≈ 10 galaxy and rapid changes in that population at an age of 500 Myr. Nature 469, 504–507 (2011). https://doi.org/10.1038/nature09717
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