Stellar systems following the R 1 / m luminosity law . II . Anisotropy , Velocity Profiles , and the FP of elliptical galaxies

Following a first paper on this subject (Ciotti 1991, hereafter Paper I), we study the dynamical properties of spherical galaxies with surface luminosity profile described by the R-law, in which a variable degree of orbital anisotropy is allowed. The parameter m for the present models covers the range [1, 10]. For these models we study the self-consistently generated phase-space distribution function (DF), and we derive – as a function of m – the minimum value of the anisotropy radius for the model consistency (i.e., in order to have a nowhere negative DF). Then we study the region in the parameter space where the R models are likely to be stable against radial-orbit instability, and we compare its size with that of the larger region corresponding to the consistency requirement. For stable anisotropic models the spatial and projected velocity dispersion profiles are obtained solving the Jeans equation, and compared to those of the globally isotropic case, already discussed in Paper I. The relevance of the results in connection with the Fundamental Plane (FP) of elliptical galaxies is pointed out: the effect on the projected velocity dispersion due to the maximum orbital anisotropy allowed by the stability requirement is well within the FP thickness, and so no fine-tuning for anisotropy is required. Finally, the Velocity Profiles are constructed as function of the projected radius and for various degrees of anisotropy, and their deviations from a gaussian discussed.