The Orbital Structure and Potential of NGC 1399

Abstract

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 -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 β = 0.3 ± 0.1 for R ≤ 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 ≈ 5 × 10 8 M ⊙ is compatible with the data in the inner 5″. The influence 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 × 10 12 M ⊙ and M/L B = 2-48(M/L) ⊙ inside R = 417″ or 35 kpc for D = 17.6 Mpc.