SIMBAD references

How is mass distributed in the Universe ? How does it compare with the distribution of light and stars ? We address these questions by examining the distribution of mass, determined from weak lensing observations, and starlight, around >105 Sloan Digital Sky Survey MaxBCG groups and clusters as a function of environment and scale, from deep inside clusters to large cosmic scales of 22h-1 Mpc. The observed cumulative mass-to-light profile, M/L(< r), rises on small scales, reflecting the increasing M/L of the central bright galaxy of the cluster, then flattens to a nearly constant ratio on scales above ∼ 300h-1 kpc, where light follows mass on all scales and in all environments. A trend of slightly decreasing M/L(r) with scale is shown to be consistent with the varying stellar population following the morphology-density relation. This suggests that stars trace mass remarkably well even though they represent only a few per cent of the total mass. We determine the stellar mass fraction and find it to be nearly constant on all scales above ∼ 300h-1 kpc, with M*/Mtot ≃ 0.01±0.004. We further suggest that most of the dark matter in the Universe is located in the large haloes of individual galaxies ( ∼ 300 kpc for L* galaxies); we show that the entire M/L(r) profile - from groups and clusters to large-scale structure - can be accounted for by the aggregate masses of the individual galaxies (whose haloes may be stripped off but still remain in the clusters), plus gas. We use the observed M/L ratio on large scales to determine the mass density of the Universe: Ω_m = 0.24 ±0.02 b_M/L^2 = 0.26 ±0.02.