PROBING THE HALO OF CENTAURUS-A - A MERGER DYNAMICAL MODEL FOR THE PNPOPULATION

Photometry and kinematics of the giant elliptical galaxy NGC 5128 (Cen taurus A) based on planetary nebulae observations (Hui et al. 1995) ar e used to build dynamical models which allow us to infer the presence of a dark matter halo. To this end, we apply a Quadratic Programming m ethod. Constant mass-to-light ratio models fail to reproduce the major axis velocity dispersion measurements at large radii: the profile of this kind of models fails off too steeply when compared to the observa tions, clearly suggesting the necessity of including a dark component in the halo. By assuming a mass-to-light ratio which is increasing wit h radius, the model satisfactorily matches the observations. The total mass for the best fit model is similar to 4 X 10(11) M. of which abou t 50% is dark matter. However; models with different total masses and dark halos are also consistent with the data; we estimate that the tot al mass of Cen A within 50 kpc may vary between 3 x 10(11) M. and 5 x 10(11) M.. The best fit model consists of 75% of stars rotating around the short axis z and 25% of stars rotating around the long axis x. Fi nally, the morphology of the projected velocity field is analyzed usin g Statler's classification criteria (Statler 1991). We find that the a ppearance of our velocity field is compatible with a type 'Nn' or 'Nd' .