Annu. Rev. Astron. Astrophys. 1991. 29: 239-274 Copyright © 1991 by Annual Reviews. All rights reserved

3.5 Mass-to-Light Ratios

The determination of mass-to-light ratios from photometric and kinematic data alone is generally not unique (45, 292, 329). Only if the run of the density profile with radius is assumed, as well as an intrinsic shape, is it possible to derive a global M / L with the virial theorem (8, 29, 38). It is impossible to derive the radial density profile from the velocity dispersion profile alone, given the freedom in the velocity anisotropy. It is nevertheless still possible to derive useful constraints on M / L with this type of information (184, 237).

One might hope that the two-dimensional velocity and velocity dispersion fields constrain the M / L value better. This needs to be verified with the help of three-integral axisymmetric and triaxial models, which show a great freedom in the velocity dispersion curves along the major and minor axis (118, 320).

Attempts to derive the radial M / L profiles usually focus on one of two issues: (a) the presence of supermassive black holes in the nuclei of galaxies, and (b) the existence of dark matter in and around elliptical galaxies. We discuss both problems separately.