Pre-heating by pre-virialization and its impact on galaxy formation

Abstract

We use recent observations of the HI mass function to constrain galaxy formation. The data conflict with the standard model where most of the gas in a low-mass dark matter halo is assumed to settle into a disc of cold gas that is depleted by star formation and supernova-driven outflows until the disc becomes gravitationally stable. Assuming a star formation threshold density supported by both theory and observations, this model predicts HI masses that are much too large. The reason is simple: supernova feedback requires star formation, which in turn requires a high surface density for the gas. Heating by the ultraviolet background can reduce the amount of cold gas in haloes with masses < 10 9.5 h-1 M⊙, but is insufficient to explain the observed HI mass function. A consistent model can be found if low-mass haloes are embedded in a pre-heated medium, with a specific gas entropy ∼10 keV cm2. In addition, such a model simultaneously matches the faint-end slope of the galaxy luminosity function without the need for any supernova-driven outflows. We propose a pre-heating model where the medium around low-mass haloes is pre-heated by gravitational pancaking. Because gravitational tidal fields suppress the formation of low-mass haloes while promoting that of pancakes, the formation of massive pancakes precedes that of the low-mass haloes within them. We demonstrate that the progenitors of present-day dark matter haloes with M ≲ 1012 h-1 M⊙ were embedded in pancakes of masses ∼5 × 10 12 h-1 M⊙ at z ∼ 2. The formation of such pancakes heats the gas to a temperature of 5 × 105 K and compresses it to an overdensity of ∼10. Such gas has a cooling time that exceeds the age of the Universe at z ≲ 2, and has a specific entropy of ∼15 keV cm2, almost exactly the amount required to explain the stellar and HI mass functions. © 2005 RAS.