NGC 5128 as an isotropic rotator

Abstract

NGC 5128 is a well-studied elliptical galaxy with an excellent kinematic data set for planetary nebulae which has been modelled up till now only by solving the Jeans equation for spherical systems. As a first approximation beyond spherical symmetry we model the galaxy as an axisymmetric system flattened by rotation with isotropic velocity distribution. We propose a new version of such an isotropic rotator having cuspy density profile with NFW-like behavior. The solutions of the Jeans equations for a single component of such form do not reproduce the data well: the rotation curve rises too slowly with radius and the velocity dispersion profile drops too fast. The data are well fitted however by a system built with two components: a more compact, flattened, less massive, fast-rotating and cold `bulge' and an extended, almost spherical, more massive, slow-rotating and hot `halo'. This picture agrees well with the results of recent N-body simulations of galaxy mergers which tend to produce oblate rotating spheroids. The total mass of the system is estimated to be (9.1 +/- 3.5) x 10^11 solar masses and the mass-to-light ratio is only 26 +/- 17 solar units.