Ionization and the Varying HI Mass Function

Abstract

If a few percent of ionizing ultraviolet photons escape from galaxies, then normal galaxies make a modest (30 to 40 percent) and spatially varying contribution to the ionizing background at low redshift (Linder et al.). Supposing that galaxies form with some total (neutral plus ionized) mass function, then the slope of the HI mass function will be less steep than the slope of the total mass function if a larger fraction of the gas in low mass galaxies becomes ionized by an ultraviolet background. If the ionizing background varies with galactic environment, then the HI mass function will also vary.