Ly{\alpha} emitters in a cosmological volume I: the impact of radiative
  transfer
  Lyman-{\alpha} emitters (LAEs) are a promising target to probe the large
scale structure of the Universe at high redshifts, $z\gtrsim 2$. However, their
detection is sensitive to radiative transfer effects that depend on local
astrophysical conditions. Thus, modeling the bulk properties of this galaxy
population remains challenging for theoretical models. Here we develop a
physically-motivated scheme to predict LAEs in cosmological simulations. The
escape of Ly{\alpha} photons is computed using a Monte Carlo radiative transfer
code which outputs a Ly{\alpha} escape fraction. To speed-up the process of
assigning escape fractions to individual galaxies, we employ fitting formulae
that approximate the full Monte Carlo results within an accuracy of 10% for a
broad range of column densities, gas metallicities and gas bulk velocities. We
apply our methodology to the semi-analytical model GALFORM on a large N-body
simulation. The Ly{\alpha} photons escape through an outflowing neutral gas
medium, implemented assuming different geometries. This results in different
predictions for the typical column density and outflow velocities of the LAE
population. To understand the impact of radiative transfer on our predictions,
we contrast our models against a simple abundance matching assignment. Our full
models populate LAEs in less massive haloes than what is obtained with
abundance matching. Overall, radiative transfer effects result in better
agreement when confronting the properties of LAEs against observational
measurements. This suggest that incorporating the effects of Ly{\alpha}
radiative transfer in the analysis of this galaxy population, including their
clustering, can be important for obtaining an unbiased interpretation of future
datasets.
