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We find that the amount and nature of the assumed ionizing background canstrongly affect galaxy formation and evolution. Galaxy evolution simulationstypically incorporate an ultraviolet background which falls off rapidly abovez=3; e.g., that of Haardt & Madau (1996). However, this decline may be toosteep to fit the WMAP constraints on electron scattering optical depth orobservations of intermediate redshift (z ~ 2-4) Ly-alpha forest transmission.As an alternative, we present simulations of the cosmological formation ofindividual galaxies with UV backgrounds that decline more slowly at highredshift: both a simple intensity rescaling and the background recently derivedby Faucher-Giguere (2009), which softens the spectrum at higher redshifts. Wealso test an approximation of the X-ray background with a similar z-dependence.We find for the test galaxies that an increase in either the intensity orhardness of ionizing radiation generically pushes star formation towards lowerredshifts: although overall star formation in the simulation boxes is reducedby 10-25%, the galaxies show a factor of ~2 increase in the fraction of starswithin a 30 kpc radius that are formed after z=1. Other effects include lategas inflows enhanced up to 30 times, stellar half-mass radii decreased by up to30%, central velocity dispersions increased up to 40%, and a strong reductionin substructure. The magnitude of the effects depends on theenvironmental/accretion properties of the particular galaxy.

The Effects of the Ionizing Radiation Background on Galaxy Evolution