ar X iv : a st ro - p h / 99 09 44 6 v 1 2 7 Se p 19 99 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 (≤ 30 km/s); the stellar velocity dispersion remains constant at 250-270 km/s. 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 percent level) the kinematic data on a grid of parametrized potentials. We find that the stellar orbital structure is moderately radial, with β = 0.3 ± 0.1 for R ≤ 60, similar to results found for some normal giant ellipticals. The gravitational potential is dominated by the luminous component out to the last data point, with a mass-to-light ratio M/LB = 10 M⊙/L⊙, although the presence of a central black hole of M ≈ 5 × 10M⊙ is compatible with the data in the inner 5 arcsec. The infludence of the dark component is marginally detected starting from R ≈ 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× 10M⊙ and M/LB = 22− 48 M⊙/L⊙ inside R = 417 or 35 kpc for D=17.6 Mpc. Subject headings: line: profiles celestial mechanics, stellar dynamics galaxies: elliptical and lenticular, cD galaxies: individual: NGC 1399 galaxies: kinematics and dynamics dark matter. Institut für Astronomie und Astrophysik, Scheinerstraße 1, D-81679 Munich, Germany Astronomisches Institut, Universität Basel, Venusstrasse 7, CH-4102 Binningen, Switzerland Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85740 Garching, Germany