Evidence for strong evolution of the cosmic star formation density at high redshifts

Deep HST/ACS and VLT/ISAAC data of the GOODS-South field were used to look for high-redshift galaxies in the rest-frame UV wavelength range and to study the evolution of the cosmic star-formation density at z similar to 7. The GOODS-South area was surveyed down to a limiting magnitude of about (J + Ks)(AB) = 25.5, looking for drop-out objects in the z'(850) ACS filter. The large sampled area would allow for the detection of galaxies that are 20 times less numerous and 1-2 mag brighter than similar studies using HST/NICMOS near-IR data. Two objects had initially been selected as promising candidates for galaxies at z similar to 7, but were subsequently dismissed and identified as Galactic brown dwarfs through 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 similar to 7 down to our limiting magnitude in the field we investigated. Our non detection of galaxies at z similar to 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 similar to 170 Myr. Our constraints also provide evidence of a significant decline in 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 what is required to completely ionize the Universe.