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Deep HST/ACS and VLT/ISAAC data of the GOODS-South field were used to lookfor high-redshift galaxies in the rest-frame UV wavelength range and to studythe evolution of the cosmic star-formation density at z~7. The GOODS-South areawas surveyed down to a limiting magnitude of about (J+Ks)=25.5 looking fordrop-out objects in the z ACS filter. The large sampled area would allow forthe detection of galaxies which are 20 times less numerous and 1-2 magnitudesbrighter than similar studies using HST/NICMOS near-IR data. Two objects wereinitially selected as promising candidates of galaxies at z~7, but havesubsequently been dismissed and identified as Galactic brown dwarfs through adetailed analysis of their morphology and Spitzer colors, as well as throughspectroscopic information. As a consequence, we conclude that there are nogalaxies 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 strongevolution of the luminosity function between z=6 and z=7, i.e. over a timeinterval of only ~170 Myr. Our constraints also provide evidence for asignificant decline of the total star formation rate at z=7, which must be lessthan 40% of that at z=3 and 40-80% of that at z=6. We also derive an upperlimit to the ionizing flux at z=7, which is only marginally consistent withthat required to completely ionize the Universe.Comment: 11 pages, A&A, in press. New version after proof correctio

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