The Mass-To-Light Function: Antibias and Omega_m
  We use large-scale cosmological simulations to estimate the mass-to-light ratio of galaxy systems as a function of scale, and compare the results with observations of galaxies, groups, clusters, and superclusters of galaxies. We find remarkably good agreement between observations and simulations. Specifically, we find that the simulated mass-to-light ratio increases with scale on small scales and flattens to a constant value on large scales, as suggested by observations. We find that while mass typically follows light on large scales, high overdensity regions --- such as rich clusters and superclusters of galaxies --- exhibit higher M/L_B values than average, while low density regions exhibit lower M/L_B values; high density regions are thus antibiased in M/L_B, with mass more strongly concentrated than blue light. This is true despite the fact that the galaxy mass density is unbiased or positively biased relative to the total mass density in these regions. The M/L_B antibias is likely due to the relatively old age of the high density regions, where light has declined significantly since their early formation time, especially in the blue band which traces recent star formation. Comparing the simulated results with observations, we place a powerful constraint on the mass density of the universe; using, for the first time, the entire observed mass-to-light function, from galaxies to superclusters, we find Omega =0.16+/-0.05. 
N.A. Bahcall, R. Cen, R. Dav\'e, J.P. Ostriker, Q. Yu (Princeton   University Observatory)