Early Preheating and Galaxy Formation

Abstract

Winds from pregalactic starbursts and 'miniquasars' may pollute the IGM with metals and raise its temperature to a high adiabat, and so inhibit the formation of early galaxies. We compute the thermal history of the IGM when it experiences a period of rapid, homogeneous "preheating" at high redshifts. Measurements of the temperature of the Lyamn-alpha forest at z~3 constrain the redshift and energy of preheating, and rule out models that preheat too late or to too high a temperature. We predict galaxy luminosity functions in preheated universes. The results depend crucially on whether the baryonic smoothing scale in the IGM is computed globally, or in a local, density-dependent fashion. Using a globally averaged smoothing scale, we find that models with excessive preheating produce too few L_* and fainter galaxies, and are therefore inconsistent with observational data. A density-dependent smoothing scale requires more energetic preheating to achieve the same degree of suppression in the faint-end slope. All models, however, appear unable to explain the sharp cut-off in the luminosity function at bright magnitudes. Supernova-driven preheating scenarios tend to raise the mean metallicity of the universe well above the minimum levels observed in the Lyman-alpha clouds. We find that ionizing photon escape fractions must be significantly higher than 10% in order to explain the low inferred HI fraction at z~6. While early preheating causes strong suppression of dwarf galaxy formation we show that it is not able to reproduce the observed abundance of satellite galaxies in the Local Group in detail.