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

The observational properties of the hydrogen Lyman-α (Lyα) emission line mean that it is the tool of choice for the study of star-forming galaxies at the highest redshifts. Doubts remain about the interpretation of the data, however, since Lyα photons scatter in the neutral interstellar medium of their host galaxies, so their sensitivity to absorption by interstellar dust may be enhanced such that the Lyα luminosity may be significantly reduced, or even completely suppressed. In order to calibrate this effect in an unbiased sample, Hayes et al. examine Lyα emissions at great distances, using a second feature, Hα. They find significant discrepancies (up to a factor of 20) in the distribution of luminosities, suggesting that only about 5% of Lyα photons escape from star-forming galaxies. This needs to be borne in mind when evaluating previous and future Lyα survey results and means that there are probably many more galaxies at these epochs waiting to be discovered.