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Using  clara  (code for Lyman alpha radiation analysis) we constrain the escape fraction of Lyman α radiation in galaxies in the redshift range  5 ≲ z ≲ 7 , based on the MareNostrum High‐ z  Universe, a smoothed particle hydrodynamics cosmological simulation with more than two billion particles. We approximate Lyman α emitters (LAEs) as dusty gaseous slabs with Lyman α radiation sources homogeneously mixed in the gas. Escape fractions for such a configuration and for different gas and dust contents are calculated using our newly developed radiative transfer code  clara . The results are applied to the MareNostrum High‐ z  Universe numerical galaxies. The model shows a weak redshift evolution and good agreement with estimations of the escape fraction as a function of reddening from observations at  z ∼ 2.2 and  z ∼ 3. We extend the slab model by including additional dust in a clumpy component in order to reproduce the ultraviolet (UV) continuum luminosity function (LF) and UV colours at redshifts   z  &gt;  rsim  5 . The LAE LF, based on the extended clumpy model, reproduces broadly the bright end of the LF derived from observations at  z ∼ 5 and  z ∼ 6. At  z ∼ 7, our model overpredicts the LF by roughly a factor of 4, presumably because the effects of the neutral intergalactic medium are not taken into account. The remaining tension between the observed and simulated faint end of the LF, both in the UV continuum and Lyman α at redshifts  z ∼ 5 and  z ∼ 6, points towards an overabundance of simulated LAEs hosted in haloes of masses 1.0 × 10 10 ≤ M h  ≤ 4.0 × 10 10   h  −1  M ⊙ . Given the difficulties in explaining the observed overabundance by dust absorption, a probable origin of the mismatch is the high star formation rate in the simulated haloes around the quoted mass range. A more efficient supernova feedback should be able to regulate the star formation process in the shallow potential wells of these haloes.

CLARA’s view on the escape fraction of Lyman α photons in high‐redshift galaxies