Mass profiles and anisotropies of early-type galaxies

We discuss the problem of using stellar kinematics of early-type galaxies t o constrain the orbital anisotropies and radial mass profiles of galaxies. We demonstrate that compressing the light distribution of a galaxy along th e line of sight produces approximately the same signature in the line-of-si ght velocity profiles as radial anisotropy, In particular, fitting spherica lly symmetric dynamical models to apparently round, isotropic face-on flatt ened galaxies leads to a spurious bias towards radial orbits in the models, especially if the galaxy has a weak face-on stellar disc. Such face-on ste llar discs could plausibly be the cause of the radial anisotropy found in s pherical models of intermediate luminosity ellipticals such as NGC 2434, 33 79 and 6703. In the light of this result, we use simple dynamical models to constrain th e outer mass profiles of a sample of 18 round, early-type galaxies. The gal axies follow a Tully-Fisher relation parallel to that for spiral galaxies, but fainter by at least 0.8 mag (I-band) for a given mass. The most luminou s galaxies show clear evidence for the presence of a massive dark halo, but the case for dark haloes in fainter galaxies is more ambiguous. We discuss the observations that would be required to resolve this ambiguity.