The angular momentum of gas in protogalaxies - II. The impact of pre-heating

We examine the effect of pre-heating of the intergalactic medium on galaxy formation using cosmological hydrodynamical simulations. By performing simulations both with and without a simple model for pre-heating, we analyse and compare the angular momentum distributions of the dark matter and the baryons. Pre-heating unbinds baryons from their dark-matter haloes, yielding a baryonic mass fraction that declines with decreasing halo mass. In addition, the spin parameter of the gas is reduced with respect to the case without pre-heating, while the misalignment between the angular momentum directions of the gas and dark matter increases strongly. The angular momentum distributions of individual haloes reveal that pre-heating decreases (increases) the mass fraction with low (negative) specific angular momentum. We discuss the implications of these findings for the formation of disc galaxies in a pre-heated intergalactic medium, and compare our results to the predictions of Maller & Dekel, who propose an alternative interpretation for the origin of the angular momentum of (proto)galaxies.