The beginning of the star formation epoch

Aimes: We intend to use the current data on large deep fields to look for high-redshift galaxies in the rest-frame UV wavelength range and study the evolution of the cosmic star-formation density at z~7. Methods: Deep HST/ACS and VLT/ISAAC data of the GOODS-South field were used to survey 130 sq.arcmin down to a limiting magnitude of about (J+Ks)AB=25.5 looking for drop-out objects in the z' ACS filter. The large sampled area would allow for the detection of galaxies which are 20 times less numerous and 1-2 magnitudes brighter than similar studies using HST/NICMOS near-IR data. Results: Two objects were initially selected as promising candidates of galaxies at z~7, but have subsequently been dismissed and identified as Galactic brown dwarfs through a detailed analysis of their morphology and Spitzer colors, as well as through spectroscopic information. As a consequence, we conclude that there are no galaxies at z~7 down to our limiting magnitude in the field we investigated. Our non detection of galaxies at z~7 provides clear evidence for a strong evolution of the luminosity function between z=6 and z=7, i.e. over a time interval of only ~170Myr. Our constraints also provide evidence for a significant decline of the total star formation rate at z=7, which must be less than 40% of that at z=3 and 40-80% of that at z=6. We also derive an upper limit to the ionizing flux at z=7, which is only marginally consistent with that required to completely ionize the Universe. This result suggests that at z=7 we are approaching the beginning of the star formation epoch