Escape of about five per cent of Lyman-α photons from high-redshift star-forming galaxies

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The Lyman-α (Lyα) emission line is the primary observational signature of star-forming galaxies at the highest redshifts1, and has enabled the compilation of large samples of galaxies with which to study cosmic evolution2,3,4,5. The resonant nature of the line, however, means that Lyα photons scatter in the neutral interstellar medium of their host galaxies, and their sensitivity to absorption by interstellar dust may therefore be greatly enhanced. This implies that the Lyα luminosity may be significantly reduced, or even completely suppressed. Hitherto, no unbiased empirical test of the escaping fraction (fesc) of Lyα photons has been performed at high redshifts. Here we report that the average fesc from star-forming galaxies at redshift z = 2.2 is just 5 per cent by performing a blind narrowband survey in Lyα and Hα. This implies that numerous conclusions based on Lyα-selected samples will require upwards revision by an order of magnitude and we provide a benchmark for this revision. We demonstrate that almost 90 per cent of star-forming galaxies emit insufficient Lyα to be detected by standard selection criteria2,3,4,5. Both samples show an anti-correlation of fesc with dust content, and we show that Lyα- and Hα-selection recovers populations that differ substantially in dust content and fesc.

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Figure 1: Observed Hα and Lyα luminosities.
Figure 2: Lyα luminosity functions.
Figure 3: Escape fraction ( fesc) and dust attenuation ( EB-V ).


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This work is based on observations made with ESO telescopes at the Paranal Observatory under programme ID 081.A-0932. The filter used to capture Lyα was financed by the Erik and Märta Holmberg foundation for astronomy and physics. M.H., D.S. and S.d.B. acknowledge the support of the Swiss National Science Foundation. G.Ö. is a Swedish Royal Academy of Sciences research fellow supported by the Knut and Alice Wallenberg foundation, and also acknowledges support from the Swedish research council (VR). J.M.M.-H. is funded by Spanish MICINN grants CSD2006-00070 (CONSOLIDER GTC) and AYA2007-67965. We thank D. Valls-Gabaud, M. Ouchi, and C. Scarlata for discussions.

Author Contributions M.H. and G.Ö. conceived the programme and manufactured the custom filter. M.H. observed, and processed and analysed the data. S.d.B. and D.S. wrote tools for analysis of the SED fitting results. A.V. produced the radiation transfer code with D.S. J.M. contributed to the use and processing of auxiliary data. All authors contributed to the interpretation of the data, the research proposal and manuscript preparation.

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Correspondence to Matthew Hayes.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This Supplementary Information file comprises: Survey redshift matching and the custom bandpass; Observations and data reduction; Photometry, selection, and catalogue assembly; SED fitting, dust extinction, and star-formation rates; Luminosity functions, Monte Carlo simulations, and escape fractions; The radiation transfer models and theoretical Lyα escape fractions, Supplementary Tables 1 and 2, Supplementary References and Supplementary Figures SI1 and SI2 with legends. (PDF 387 kb)

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Hayes, M., Östlin, G., Schaerer, D. et al. Escape of about five per cent of Lyman-α photons from high-redshift star-forming galaxies. Nature 464, 562–565 (2010) doi:10.1038/nature08881

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