THE OPTICALLY DARK SIDE OF GALAXY FORMATION

Deep optical survey(1,2) probe the rest-frame ultraviolet luminosities of high-redshift galaxies, They can therefore be used to infer star f ormation rates, under assumptions about young stellar populations, Cur rent data suggest that the global star-formation rate of the Universe peaked at a redshift of z=1, then subsequently declined(3,4), leading to claims that the bulk of star formation in the Universe has been see n, However, the large uncertainties inherent in correcting for ultravi olet absorption by dust associated with young stars suggest that these formation rates might be substantially underestimated in high-redshif t galaxies. Here we circumvent this problem by considering the bust th ermal emission at infrared (and submillimetre) wave-lengths. We propos e an improved determination of the long-sought cosmic infrared backgro und(5) (built up from the accumulated infrared light of faint galaxies along the line of sight), from which we are able to estimate the requ ired population of high-redshift, dust-enshrouded starburst galaxies, We argue that most of the star formation at high redshift may be hidde n by dust, and we define the necessary characteristics of a feasible f ar-infrared survey that could detect this population.