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A. J. Benson (California Institute of Technology, U.S.A.), C. G. Lacey (SISSA, Italy), C. S. Frenk, S. Cole, C. M. Baugh (University of Durham, UK)
We develop a model for the evolution of the global properties of the intergalactic medium (IGM), specifically the ionization states of hydrogen and helium and the volume averaged temperature, in the presence of a photoionizing background due to stars and quasars. Our model is able to reproduce many of the results of numerical simulations following the same physical processes. We use this model to predict the thermodynamic history of the IGM when photoionized by galaxies forming in a cold dark matter (CDM) universe. We then consider how the hot IGM and photoionizing background act to inhibit galaxy formation in low-mass dark matter halos thereby producing a self-consistent model of galaxy formation and the IGM. The IGM is reheated twice (during reionization of H{\sc i} and He{\sc ii}) and we find that the star formation rate per unit volume is slightly suppressed after each episode of reheating. We find that galaxies brighter than L* are mostly unaffected by reionization, while the abundance of faint galaxies is significantly reduced, leading to luminosity functions with shallow faint end slopes in reasonable agreement with recent observational data. Reionization also affects other properties of these faint galaxies, in a readily understandable way. Supplementing our model with a detailed treatment of the evolution of satellite galaxies, including tidal and dynamical friction forces due to the environment in which they orbit, we consider the population of satellites around Milky Way-like galaxies. We find that reionization significantly reduces the abundance of these satellites, predicting numbers consistent with those observed.
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