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2.1 Galaxies and Clusters: Dynamical Measures & Mass-to-Light Ratios

The contribution of galaxies to the mass density can be determined by integrating the luminosity function per unit volume for galaxies and multiplying by an (assumed, constant) mean mass-to-light (M / L) ratio. The dynamical masses of galaxies can be determined from rotation curves for spiral galaxies, or the measurement of velocity dispersions and application of the virial theorem both for individual elliptical galaxies. The latter method can also be applied for groups and clusters of galaxies (as Zwicky did in the 1930's).

This method has several advantages. First it is conceptually simple and model-independent. Unlike some of the global techniques discussed below, this method is independent of both H0 and OmegaLambda. However, there are a number of underlying assumptions. Most important is the assumption that galaxies trace all mass. In addition, there are implicit, underlying assumptions concerning the similarity of mass-to-light ratios in different systems (ignoring, for example, potential differences in initial mass functions, star formation histories, dark remnant populations, dust content, etc.) The estimates based on this method tend to yield low values of Omegam of leq 0.25.