The submillimetre extragalactic background light and the star-formation history of the Universe

The submillimetre extragalactic background light is comparable with or exce eds that of the optical and ultraviolet (UV) wavelength ranges, showing dir ectly that much of the energy radiated by star formation and active galacti c nuclei is moved to far-infrared wavelengths. However, it is only as this background at 850 mu m has been resolved with direct submillimetre imaging that we have seen that it is largely created by a population of ultralumino us (or near-ultraluminous) infrared galaxies, which appear to lie at relati vely high redshifts (z > 1). Mapping the redshift evolution of this major p ortion of universal star formation has been difficult because of the poor s ubmillimetre spatial resolution, but this difficulty can be overcome by usi ng extremely deep centimetre continuum radio observations to obtain precise astrometric information, since the bulk of the brighter submillimetre sour ces have detectable radio counterparts. With this precise position informat ion available, we find that most of the submillimetre sources are extremely faint in the optical and near-infrared (I much greater than 24 and K = 21- 22) and inaccessible to optical spectroscopy. Rough photometric redshift es timates can be made from combined radio and submillimetre energy distributi ons. We shall refer to this procedure as millimetric redshift estimation to distinguish it from photometric estimators in the optical and near-infrare d. These estimators place the bulk of the submillimetre population at z = 1 -3, where it corresponds to the high-redshift tail of the faint centimetre radio population. While still preliminary, the results suggest that the sub millimetre population appears to dominate the star formation in this redshi ft range by almost an order of magnitude over the mostly distinct populatio ns selected in the optical/UV.