Simulated vs. observed UV emission at high redshift: a hint for a clumpy
  ISM?
  We discuss the rest-frame UV emission between 5< z < 7 from the MareNostrum
High-z Universe, a SPH simulation done with more than 2 billion particles.
Cosmological simulations of galaxy formation generally overpredict the UV
restframe luminosity function at high redshift, both at the bright and faint
ends. In this Letter we explore a dust attenuation model where a larger
extinction is applied to star populations younger than a given age, mimicking
the effect of a clumpy interstellar medium. We show that this scenario fits
reasonably well both the UV luminosity functions and the UV-continuum slopes
derived from observations. The model assumes a large obscuration for stars
younger than 25 Myr from the gas clouds where they should be embedded at their
formation time. We find that the optical depth in these clouds should be
between 30 and 100 times larger than the mean optical depth for the homogeneous
part of the interstellar medium. These values are one order of magnitude larger
than those estimated in local galaxies. Therefore, we conclude that LambdaCDM
predictions for the high-z UV emission can accommodate the current observations
if we consider a dust extinction model based on the assumption of a clumpy
environment at high redshift.
