  Radiation feedback is typically implemented using subgrid recipes in
hydrodynamical simulations of galaxies. Very little work has so far been
performed using radiation-hydrodynamics (RHD), and there is no consensus on the
importance of radiation feedback in galaxy evolution. We present RHD
simulations of isolated galaxy disks of different masses with a resolution of
18 pc. Besides accounting for supernova feedback, our simulations are the first
galaxy-scale simulations to include RHD treatments of photo-ionisation heating
and radiation pressure, from both direct optical/UV radiation and
multi-scattered, re-processed infrared (IR) radiation. Photo-heating smooths
and thickens the disks and suppresses star formation about as much as the
inclusion of ("thermal dump") supernova feedback does. These effects decrease
with galaxy mass and are mainly due to the prevention of the formation of dense
clouds, as opposed to their destruction. Radiation pressure, whether from
direct or IR radiation, has little effect, but for the IR radiation we show
that its impact is limited by our inability to resolve the high optical depths
for which multi-scattering becomes important. While artificially boosting the
IR optical depths does reduce the star formation, it does so by smoothing the
gas rather than by generating stronger outflows. We conclude that although
higher-resolution simulations, and potentially also different supernova
implementations, are needed for confirmation, our findings suggest that
radiation feedback is more gentle and less effective than is often assumed in
subgrid prescriptions.
