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Despite recent success in forming realistic present-day galaxies, simulationsstill form the bulk of their stars earlier than observations indicate. Weinvestigate the process of stellar mass assembly in low-mass field galaxies, adwarf and a typical spiral, focusing on the effects of radiation from youngstellar clusters on the star formation (SF) histories. We implement a novelmodel of SF with a deterministic low efficiency per free-fall time, as observedin molecular clouds. Stellar feedback is based on observations of star-formingregions, and includes radiation pressure from massive stars, photoheating in HII regions, supernovae and stellar winds. We find that stellar radiation has astrong effect on the formation of low-mass galaxies, especially at z > 1, whereit efficiently suppresses SF by dispersing cold and dense gas, preventingrunaway growth of the stellar component. This behaviour is evident in a varietyof observations but had so far eluded analytical and numerical models withoutradiation feedback. Compared to supernovae alone, radiation feedback reducesthe SF rate by a factor of ~100 at z < 2, yielding rising SF histories whichreproduce recent observations of Local Group dwarfs. Stellar radiation alsoproduces bulgeless spiral galaxies and may be responsible for excess thickeningof the stellar disc. The galaxies also feature rotation curves and baryonfractions in excellent agreement with current data. Lastly, the dwarf galaxyshows a very slow reduction of the central dark matter density caused byradiation feedback over the last ~7 Gyr of cosmic evolution.

Low-mass galaxy assembly in simulations: regulation of early star formation by radiation from massive stars