Scale-free dynamical models for galaxies: flattened densities in spherical potentials

Abstract

This paper presents two families of phase-space distribution functions (DFs) that generate scale-free spheroidal mass densities in scale-free spherical potentials. The `case I' DFs are anisotropic generalizations of the flattened f(E,L_z) model, which they include as a special case. The `case II' DFs generate flattened constant-anisotropy models, in which the constant ratio of rms tangential to radial motion is characterized by Binney's parameter beta. The models can describe the outer parts of galaxies and the density cusp structure near a central black hole, but also provide general insight into the dynamical properties of flattened systems. The dependence of the intrinsic and projected properties on the model parameters and the inclination is described. The observed ratio of the rms projected line-of-sight velocities on the projected major and minor axes of elliptical galaxies is best fit by the case II models with beta > 0. These models also predict non-Gaussian velocity profile shapes consistent with existing observations. The distribution functions are used to model the galaxies NGC 2434 (E1) and NGC 3706 (E4), for which stellar kinematical measurements out to two effective radii indicate the presence of dark halos (Carollo et al.). The velocity profile shapes of both galaxies can be well fit by radially anisotropic case II models with a spherical logarithmic potential. This contrasts with the f(E,L_z) models studied previously, which require flattened dark halos to fit the data.

LaTeX file, uses standard macros mn.sty, epsf.sty. 17 pages including 7 figures

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