Escape of about five per cent of Lyman-alpha photons from high-redshift
  star-forming galaxies
  The Lyman-alpha (Lya) emission line is the primary observational signature of
star-forming galaxies at the highest redshifts, and has enabled the compilation
of large samples of galaxies with which to study cosmic evolution. The resonant
nature of the line, however, means that Lya photons scatter in the neutral
interstellar medium of their host galaxies, and their sensitivity to absorption
by interstellar dust may therefore be enhanced greatly. This implies that the
Lya luminosity may be significantly reduced, or even completely suppressed.
Hitherto, no unbiased empirical test of the escaping fraction (f_esc) of Lya
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 Lya and Ha. This implies that numerous
conclusions based on Lya-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 Lya to be
detected by standard selection criteria. Both samples show an anti-correlation
of fesc with dust content, and we show that Lya- and Ha-selection recovers
populations that differ substantially in dust content and fesc.
