Cold gas in dark matter halos and the formation of late-type galaxies

Abstract

We use recent observations of the HI mass function to constrain the amount of cold gas in dark matter halos. It is found that the cold gas mass in a halo decreases rapidly with decreasing halo mass for low-mass halos with M< 10 12 h −1 M� . This result is in conflict with the standard model, in which most of the gas in a low-mass halo is assumed to settle into a gaseous disk, and the cold gas is depleted by star formation and supernova-driven outflow until the disk becomes gravitationally stable. Heating by the UV background can reduce the amount of cold gas in halos with masses < 10 10 h −1 M� , but is insufficient to explain the observational result. A consistent model can be found if low-mass halos are embedded in a preheated medium, with a specific gas entropy ∼ 10 keV cm 2 . Such a model can also explain why the faint-end slope of the galaxy luminosity function is flat. We propose a preheating model in which the medium around low-mass halos is preheated by gravitational pancaking. Since large-scale tidal fields tend to suppress the formation of low-mass virialised halos while promoting the formation of pancakes, the formation of massive pancakes may precede that of low-mass dark halos at the present time. 'Previrialisation' of such pancakes can heat the intergalactic medium. The preheated gas has a cooling time longer than the age of the Universe at z ∼ 2, and has a specific entropy comparable to that required to reduce the amount of cold gas and star formation activity in galaxy halos.