Why are low mass halos not forming stars?

The mass to light ratio of galaxies is minimum for a Galactic mass halo (~1.e12 Msun). For both smaller and larger halo masses the stellar mass, and hence the luminosity, decreases relative to the halo mass. At larger halo masses star formation is believed to be quenched due to AGN feedback, and at lower halo masses momentum feedback by supernovae has been blamed for suppressing star formation. This paper (http://arxiv.org/abs/1105.2376) suggests an alternate possibility for explaining lower star formation efficiency in smaller mass halos (<few 1.e9 Msun; to quench star-formation in galaxies larger than this still requires supernova feedback). It suggests that lower mass halos (with low metallicities and smaller gas column densities) cannot form molecular hydrogen, which is a pre-requisite to form stars according to recent observations and modeling. This is a simulation paper which uses a simplified equilibrium model for molecular gas fraction depending on the UV background, metallicity, dust, neutral hydrogen density, etc. and calculates the star formation rate based on molecular hydrogen fraction. Using this prescription the authors are able to get the reduction in stellar mass fraction in halo masses less than few times 1.e9 solar masses. Their model also predict neutral hydrogen in lower mass halos because it has not been converted in the molecular phase or stars.