Star formation and clumps in cosmological galaxy simulations with radiation pressure feedback

Cosmological simulations of galaxies have typically produced too many stars at early times. We study the global and morphological effects of radiation pressure (RP) in eight pairs of high-resolution cosmological galaxy formation simulations. We find that the additional feedback suppresses star formation globally by a factor of similar to 2. Despite this reduction, the simulations still overproduce stars by a factor of similar to 2 with respect to the predictions provided by abundance matching methods for haloes more massive than 5 x 10(11)M(circle dot) h(-1). We also study the morphological impact of RP on our simulations. In simulations with RP the average number of low-mass clumps falls dramatically. Only clumps with stellar masses M-clump/M-disc <= 5 per cent are impacted by the inclusion of RP, and RP and no-RP clump counts above this range are comparable. The inclusion of RP depresses the contrast ratios of clumps by factors of a few for clump masses less than 5 per cent of the disc masses. For more massive clumps, the differences between and RP and no-RP simulations diminish. We note, however, that the simulations analysed have disc stellarmasses below about 2x10(10)M(circle dot) h(-1). By creating mock Hubble Space Telescope observations we find that the number of clumps is slightly reduced in simulations with RP. However, since massive clumps survive the inclusion of RP and are found in our mock observations, we do not find a disagreement between simulations of our clumpy galaxies and observations of clumpy galaxies. We demonstrate that clumps found in any single gas, stellar, or mock observation image are not necessarily clumps found in another map, and that there are few clumps common to multiple maps.