Sensitive optical surveys have revealed a large population of `faint blue galaxies' which are believed to be young galaxies, observed close to their time of formation. But there has been considerable uncertainty regarding the epochs at which these galaxies are observed, owing to the difficulties inherent in determining spectroscopic redshifts for very faint objects. Here, by modelling the counts and colours of galaxies at the faintest detection limits, we show that the faint blue galaxies are likely to lie at high redshift (z approx. 2). This conclusion holds regardless of whether the Universe is assumed to be open or at the critical density (flat). In an open universe, the data are consistent with galaxy models in which star formation rates decay exponentially with decreasing redshift, whereas the assumption of a flat universe requires the addition of a population of galaxies which are seen only at high redshift.

16 pages, LaTeX, uses nature.sty, 3 postscript figures plus 1 gif figure; to appear on Sept 19th in Nature 383, p326; also available from http://star-www.dur.ac.uk/~hjmcc/herschel