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Letters to Nature
Nature 362, 324 - 326 (25 March 1993); doi:10.1038/362324a0

A young source of optical emission from distant radio galaxies

F. Hammer*, O. Le Fèvre* & M. C. Angonin*

*Département d'Astrophysique Extragalactique et Cosmologie, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France
Canada–France–Hawaii Telescope Corporation, PO Box 1597, Kamuela, Hawaii 96743, USA

DISTANT radio galaxies provide valuable insights into the properties of the young Universe—they are the only known extended optical sources at high redshift and might represent an early stage in the formation and evolution of galaxies in general. This extended optical emission often has very complex morphologies, but the origin of the light is still unclear. Here we report spectroscopic observations for several distant radio galaxies (0.75≤ z ≤ 1.1) in which the rest-frame spectra exhibit featureless continua between 2,500 Å and 5,000 Å. We see no evidence for the break in the spectrum at 4,000 Å expected for an old stellar population1–3, and suggest that young stars or scattered emissions from the active nuclei are responsible for most of the observed light. In either case, this implies that the source of the optical emission is com-parable in age to the associated radio source, namely 107 years or less.

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