Hyperluminous infrared galaxies

39 galaxies are now known, from follow-up of faint IRAS sources and from submillimetre observations of high-redshift AGN, with far-infrared luminosities > 10(13) L.. 13 of these, which have been found in 60- or 850-mu m surveys, 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. In this paper I argue, on the basis of detailed modelling of the spectral energy distributions of hyperluminous galaxies with accurate radiative transfer models, and from evidence of high gas mass in several cases, that the bulk of the emission from these galaxies at rest frame wavelengths greater than or equal to 50 mu m is caused by star formation. Even after correction for the effects of lensing, hyperluminous galaxies with emission peaking at rest frame wavelengths greater than or equal to 50 mu m are therefore undergoing star formation at rates > 10(3) M. yr(-1) and are strong candidates for being primeval galaxies, in the process of a major episode of star formation.