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3.1. Mass to Light Ratio

The mass-to-light ratio M/L is measured from virialized systems from galaxies up to rich clusters. The asymptotic universal value of the luminosity density, curly, is estimated by integrating the galaxy luminosity function. The mass-to-light ratio in clusters is assumed to be the same as outside clusters. Then

Equation 8 (8)

New Developments: Better masses for clusters and groups are now available from virial analysis of galaxies, from X-ray gas with resolved temperature structure, and from gravitational lensing. bullet Masses for extended galaxy halos are also becoming available from satellite galaxies and from lensing.

Pro: This is a simple, straightforward method. The result is independent of H0.

Con: The basic hypothesis is unjustified. It rests on the implicit assumption that galaxies form in an unbiased way. Clusters and groups may have extended dark halos that are not traced by the galaxies or by the X-ray gas, so M/L may vary within the clusters as a function of radius. This problem is even worse within galactic halos. bullet The total mass of clusters may be underestimated in the virial analysis based on galaxy velocities because of possible velocity antibiasing, and because of applying a spherical analysis to elongated systems. bullet The mass based on X-ray measurements may be underestimated by the assumption that the X-ray gas is in hydrostatic equilibrium. The data allows significant freedom in the mass profile, which translates to an uncertainty of factor two within the Abell radius, and a larger error at larger radii [18]. bullet The luminosity density may not be contributed by the same stars that dominate the cluster light. There are factors of 2-3 uncertainties in curly stemming, for example, by differences in overall metallicity and extinction by dust.

Current Results: From virial analysis of galaxy velocity dispersions in several clusters stuck together, and a corresponding estimate of curly: Omegam appeq 0.25 ± 0.05 [19].

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