Abstract
Spectroscopic measurements of a galaxy that shines brightly at submillimetre wavelengths place it in the middle of a nascent galaxy cluster at a scant one billion years after the Big Bang. See Letter p.233 The brightest sub-millimetre radio source in the Hubble Deep Field view of the distant Universe, known as HDF 850.1, has proved enigmatic, evading detection in the optical and near-infrared ranges despite an intensive search. Without the discovery of a counterpart at shorter wavelengths, it has not been possible to estimate the source's redshift, size or mass directly. Now, by using a millimetre-wave molecular line scan, the redshift of HDF 850.1 has been determined. At z ≈ 5.2, it is much higher than expected and corresponds to a cosmic age of only 1.1 billion years after the Big Bang. Calculations from the new data suggest a high annual star formation rate of 850 solar masses and a mass of 1.3 × 1011 solar masses. But as yet there is no sign of a starlight-emitting counterpart.
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