THE question of when galaxies and stars started to form in the early Universe is one of the most fundamental in cosmology. Faint radio sources (with fluxes of a few microjanskys or less) may be these early galaxies; radio surveys have shown them to be increasingly common as fainter levels are reached1-7, suggesting that they are an important constituent of the early Universe. Many of these sources were identified with faint blue galaxies—some of which have peculiar morphology (perhaps arising from inter actions) 1, 8-10or with galaxies hosting enhanced star formation activity10-12, but lack of optical resolution has made the nature of these identifications rather uncertain. Here we present the results of a very deep radio survey, obtained with the Very Large Array, of a field imaged with the Wide Field Camera on the Bubble Space Telescope. We find that most of the microjansky sources are indeed faint blue galaxies, with light profiles resembling disk galaxies. More than half of these galaxies occur in pairs or small groups, compared to fewer than ten per cent of nearby galaxies. We conclude that the microjansky sources are luminous disk galaxies, in a starburst (or post-starburst) phase that was triggered by interactions or mergers. Only a few show evidence for an active nucleus2, 13, 14, which is the other possible power source for the emission.
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