Making Galaxies In a Cosmological Context: the need for early stellar feedback

We introduce the Making Galaxies In a Cosmological Context (MAGICC) programme of smoothed particle hydrodynamics simulations. We describe a parameter study of galaxy formation simulations of an L* galaxy that uses early stellar feedback combined with supernova feedback to match the stellar mass-halo mass relationship. While supernova feedback alone can reduce star formation enough to match the stellar mass-halo mass relationship, the galaxy forms too many stars before z = 2 to match the evolution seen using abundance matching. Our early stellar feedback is purely thermal and thus operates like an ultraviolet ionization source as well as providing some additional pressure from the radiation of massive, young stars. The early feedback heats gas to >10(6) K before cooling it to 10(4) K. The pressure from this hot gas creates a more extended disc and prevents more star formation prior to z = 1 than supernova feedback alone. The resulting disc galaxy has a flat rotation curve, an exponential surface brightness profile, and matches a wide range of disc scaling relationships. The disc forms from the inside-out with an increasing exponential scale length as the galaxy evolves. Overall, early stellar feedback helps to simulate galaxies that match observational results at low and high redshifts.