Cosmological simulations of disk galaxy formation, when compared to the observed Tully–Fisher relation, suggest a low mass to light (M/L) ratio for the stellar component in spirals. We show that a number of 'bottom-light' initial mass functions (IMFs) suggested independently in the literature, do imply M/L ratios as low as required, at least for late type spirals (Sbc–Sc). However the typical M/L ratio, and correspondingly the zero point of the Tully–Fisher relation, is expected to vary considerably with Hubble type.
Bottom-light IMFs tend to have a metal production in excess of what is typically estimated for spiral galaxies. Suitable tuning of the IMF slope and mass limits, post-supernova fallback of metals onto black holes or metal outflows must then be invoked, to reproduce the observed chemical properties of disk galaxies.