The distribution of stellar orbits in the giant elliptical galaxy NGC 2320

We present direct observational constraints on the orbital distribution of the stars in the giant elliptical NGC 2320. Long-slit spectra along multipl e position angles are used to derive the stellar line-of-sight velocity dis tribution within one effective radius, R-eff. In addition, the rotation cur ve and dispersion profile of an ionized gas disk are measured from the [O I II] emission lines. After correcting for the asymmetric drift, we derive th e circular velocity of the gas, which provides an independent constraint on the gravitational potential. To interpret the stellar motions, we build ax isymmetric three-integral dynamical models based on an extension of the Sch warzschild orbit-superposition technique. We consider two families of gravi tational potential, one in which the mass follows the light (i.e., no dark matter) and one with a logarithmic gravitational potential. Using chi(2) st atistics, we compare our models to both the stellar and gas data to constra in the value of the V-band mass-to-light ratio, gamma(V). We find gamma(V) = 15.0 +/- 0.6 h(75) for the mass-follows-light models and gamma(V) = 17.0 + 0.7 h(75) for the logarithmic models. For the latter, gamma(V) is defined within a sphere of 15 " radii. Models with radially constant gamma(V) and logarithmic models with dark matter provide comparably good fits to the dat a and possess similar dynamical structure. Across the full range of gamma(V ) permitted by the observational constraints, the models are radially aniso tropic in the equatorial plane over the radial range of our kinematical dat a (1 " less than or similar to r less than or similar to 40 "). Along the t rue minor axis, they are more nearly isotropic. The best fitting model has sigma(r)/sigma(total) similar or equal to 0.7, sigma(phi)/sigma(total) simi lar or equal to 0.5-0.6, and sigma(theta)/sigma(total) similar or equal to 0.5 in the equatorial plane.