The orbital structure and potential of NGC 1399

Accurate and radially extended stellar kinematic data reaching R = 97 from the center are presented for the cD galaxy of Fornax NGC 1399. The stellar rotation is small (less than or equal to 30 km s(-1)); the stellar velocity dispersion remains constant at 250-270 km s(-1). The deviations from Gaussian line-of-sight velocity distributions are small, at the percent level. We construct dynamical models of the galaxy, deprojecting its nearly round (E0-E1) surface brightness distribution and determining the spherical distribution function that best fits (at the 4% level) the kinematic data on a grid of parameterized potentials. We find that the stellar orbital structure is moderately radial, with beta = 0.3 +/- 0.1 for R less than or equal to 60 , similar to results found for some normal giant elliptical galaxies. The gravitational potential is dominated by the luminous component out to the last data point, with a mass-to-light ratio M/L-B = 10(M/L)., although the presence of a central black hole of M approximate to 5 x 10(8) M. is compatible with the data in the inner 5 . The influence of the dark component is marginally detected starting from R approximate to 60 . Using the radial velocities of the globular clusters and planetary nebulae of the galaxy, we constrain the potential more strongly, ruling out the self-consistent case and finding that the best-fit solution agrees with X-ray determinations. The resulting total mass and mass-to-light ratio are M = 1.2-2.5 x 10(12) M. and M/L-B = 2-48(M/L). inside R = 417 or 35 kpc for D = 17.6 Mpc.