Early Preheating and Galaxy Formation

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Abstract
Pregalactic starbursts and `miniquasars' may raise the temperature of the IGM to a higher adiabat than photoionization, and so inhibit the formation of galaxies. We compute the thermal history of the IGM when it experiences such "preheating", and the impact of this on the IGM ionization state and subsequent galaxy formation. The temperature of the Lyman-alpha forest at redshift z~3 constrains the redshift and energy of preheating, and rules out models that preheat too late or to too high a temperature. We predict galaxy luminosity functions in preheated universes. The results depend on whether the baryonic smoothing in the IGM is computed globally, or in a local, density-dependent fashion. Using a globally averaged smoothing, we find that models with excessive preheating produce too few L* and fainter galaxies, and are therefore inconsistent with observational data. More moderate preheating scenarios, with T_IGM >~ 10^5 K at z~10, are able to flatten the faint-end slope of the luminosity function without the need for any local feedback mechanism within galaxies. A density-dependent smoothing scale requires more energetic preheating to achieve the same degree of suppression. Preheating causes a sharp drop in the abundance of neutral hydrogen in the IGM and is often sufficient to double ionize helium. Ionizing photon escape fractions must be significantly higher than 10% to explain the low inferred HI fraction at z~6. While early preheating causes strong suppression of dwarf galaxy formation it is not able to reproduce the observed abundance of satellite galaxies in the Local Group in detail.

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