Hyperluminous infrared galaxies

39 galaxies are now known, from follow-up of faint IRAS sources and from su bmillimetre observations of high-redshift AGN, with far-infrared luminositi es > 10(13) L.. 13 of these, which have been found in 60- or 850-mu m surve ys, form an important unbiased subsample. 12 have been found by comparison of 60-mu m surveys with quasar or radio galaxy catalogues, or from infrared surveys with colour selection biased towards AGN, while a further 14 have been found through submillimetre observations of known high-redshift AGN. I n this paper I argue, on the basis of detailed modelling of the spectral en ergy distributions of hyperluminous galaxies with accurate radiative transf er models, and from evidence of high gas mass in several cases, that the bu lk of the emission from these galaxies at rest frame wavelengths greater th an or equal to 50 mu m is caused by star formation. Even after correction f or the effects of lensing, hyperluminous galaxies with emission peaking at rest frame wavelengths greater than or equal to 50 mu m are therefore under going star formation at rates > 10(3) M. yr(-1) and are strong candidates f or being primeval galaxies, in the process of a major episode of star forma tion.