High-redshift Lyman-α (Lyα) blobs1,2 are extended, luminous but rare structures that seem to be associated with the highest peaks in the matter density of the Universe3,4,5,6. Their energy output and morphology are similar to those of powerful radio galaxies7, but the source of the luminosity is unclear. Some blobs are associated with ultraviolet or infrared bright galaxies, suggesting an extreme starburst event or accretion onto a central black hole8,9,10. Another possibility is gas that is shock-excited by supernovae11,12. But not all blobs are associated with galaxies13,14, and these ones may instead be heated by gas falling into a dark-matter halo15,16,17,18,19. The polarization of the Lyα emission can in principle distinguish between these options20,21,22, but a previous attempt to detect this signature returned a null detection23. Here we report observations of polarized Lyα from the blob LAB1 (ref. 2). Although the central region shows no measurable polarization, the polarized fraction (P) increases to ∼20 per cent at a radius of 45 kiloparsecs, forming an almost complete polarized ring. The detection of polarized radiation is inconsistent with the in situ production of Lyα photons, and we conclude that they must have been produced in the galaxies hosted within the nebula, and re-scattered by neutral hydrogen.
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We thank P. Ogle and J. Colbert for valuable suggestions about polarimetric observations; N. Panagia and T. Jones for comments on the manuscript; and D. Schaerer, N. Scoville, C. Lidman, A. Dey, M. Prescott and P. Lynam for discussions. This work was based on observations made with European Southern Observatory telescopes at the Paranal Observatory under programme ID 084.A-0954. M.H. was supported in part by the Swiss National Science Foundation, and also received support from Agence Nationale de la Recherche (reference ANR-09-BLAN-0234-01).
The authors declare no competing financial interests.
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Hayes, M., Scarlata, C. & Siana, B. Central powering of the largest Lyman-α nebula is revealed by polarized radiation. Nature 476, 304–307 (2011). https://doi.org/10.1038/nature10320
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