  The effect of the cosmic UV background on star formation in high redshift
galaxies is explored by means of high resolutions cosmological simulations. The
simulations include star formation, 3D radiative transfer, and a highly
detailed ISM model, and reach spatial resolution sufficient to resolve
formation sites for molecular clouds. In the simulations the local radiation
field in the Lyman-Werner band around star-forming molecular clouds dominates
over the cosmic UV background by a factor of 100, similarly to the interstellar
radiation field in the Milky Way and in a few high redshift galaxies for which
measurements exist. The cosmic UV background, therefore, is essentially
irrelevant for star formation in normal galaxies.
