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) data have yielded the first reliable detections of z ~ 8 galaxies that, together with reports of a gamma-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. 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 ~ 8 to z ~ 10. The 100-200 Myr before z ~ 10 is clearly a crucial phase in the assembly of the earliest galaxies.
We provide some extra information (under embargo until wednesday 7pm Dutch time) on our paper. You can find the paper as it will appear in Nature and the supplemental information (which will appear on Nature only in electronic form). The supplemental information contains all the details of the analysis.