Probing the halo of Centaurus A: a merger dynamical model for the PN population

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Photometry and kinematics of the giant elliptical galaxy NGC 5128 (Centaurus A) based on planetary nebulae observations (Hui 1995) are 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 method. 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 falls off too steeply when compared to the observations, clearly suggesting the necessity of including a dark component in the halo. By assuming a mass-to-light ratio which is increasing with radius, the model satisfactorily matches the observations. The total mass for the best fit model is 4×10¹¹M of which about 50% is dark matter. However, models with different total masses and dark halos are also consistent with the data; we estimate that the total mass of Cen A within 50 kpc may vary between 3×10¹¹M and 5×10¹¹M. 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. Finally, the morphology of the projected velocity field is analyzed using Statler's classification criteria (Statler 1991). We find that the appearance of our velocity field is compatible with a type 'Nn' or 'Nd'.