• galaxies: evolution;
  • galaxies: high-redshift


We present theoretical results for the expected fraction of Lyman break galaxies (LBGs) to be detected as strong Lyman α emitters (LAEs) in the redshift range 5 ≤ z ≤ 7. We base our analysis on the 2-billion particle smoothed particle hydrodynamics simulation MareNostrum High-z Universe. We approximate galaxies as static dusty slabs with an additional clumpy dust distribution affecting stellar populations younger than 25 Myr. The model for the Lyman α escape fraction is based on the results of our Monte Carlo radiative transfer code (clara) for a slab configuration. We also fix the transmission of Lyman α photons through the intergalactic medium to a constant value of 50 per cent at all redshifts. From the results of this model, we calculate xLyα, the fraction of LBGs with Lyα equivalent width larger than 50 Å. We find a remarkable agreement with observational data at 4.5 < z < 6. For bright (−22 < MUV < −20.5) and faint (−20.5 < MUV < −18.5) galaxies, our model predicts xLyα= 0.02 ± 0.01 and 0.47 ± 0.01, while observers report xLyα= 0.08 ± 0.02 and 0.47 ± 0.16, respectively. Additional evolution of the extinction model at redshift z ∼ 7, which decreases the intensity of transmitted Lyman α radiation by a factor of fT= 0.4 so as to match the LAE luminosity function at z ∼ 6.5, naturally provides a good match for the recently reported xLyα fractions at z > 6.3. Exploring different toy models for the Lyman α escape fraction, we show that a decreasing Lyman α escape fraction with increasing ultraviolet galaxy luminosity is a key element in our model to explain the trend of larger xLyα fractions for fainter LBGs.