Lyman-break galaxies at z ~ 5 - I. First significant stellar mass assembly in galaxies that are not simply z ~ 3 LBGs at higher redshift

International audienceWe determine the ensemble properties of z ~ 5 Lyman-break galaxies (LBGs) selected as V-band dropouts to iAB < 26.3 in the Chandra Deep Field-South using their rest-frame UV-to-visible spectral energy distributions. By matching the selection and performing the same analysis that has been used for z ~ 3 samples, we show clear differences in the ensemble properties of two samples of LBGs which are separated by 1Gyr in look-back time. We find that z ~ 5 LBGs are typically much younger (~70 per cent), whereas they compris