Next Contents Previous


Everything presented so far in this review has dealt with the luminosity function of galaxies, which is not the same as the mass function (by mass function here I am referring to total mass, where the dark matter content of individual galaxies is included). The two, however, probably are fairly similar, unless there exist a significant dispersion in the mass-to-light vs. light distribution function for galaxies in clusters. Clusters of galaxies possess a great deal of dark matter so that it is not impossible that they contain many galaxies that are almost completely dark. The mass function would be a useful probe of the fluctuation spectrum, since it bypasses all the complex physics of the baryonic component outlined in Section 1 (in the cores of rich clusters, however, there remains the caveat that the dark-matter properties of galaxies might be significantly affected by cluster-related processes e.g. tidal stripping).

The mass function is not straightforward to measure. The best hope is to determine it through gravitational lensing measurements, for example (i) galaxy-galaxy strong lensing, or (ii) the Natarajan-Kneib [24] method, which attributes granularity in the weak lensing shear map to lensing by individual galaxy halos. In the upcoming age of 8 and 10 m telescopes, it is likely that these kinds of measurements will provide important constraints.