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 ~2. Despite this reduction, the simulations still
overproduce stars by a factor of ~2 with respect to the predictions provided by
abundance matching methods for halos more massive than 5E11 Msun/h (Behroozi,
Wechsler & Conroy 2013).
  We also study the morphological impact of radiation pressure on our
simulations. In simulations with RP the average number of low mass clumps falls
dramatically. Only clumps with stellar masses Mclump/Mdisk <= 5% 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% of the disk masses. For more
massive clumps, the differences between and RP and no-RP simulations diminish.
We note however, that the simulations analyzed have disk stellar masses below
about 2E10 Msun/h.
  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.
