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 multiple position angles are used to derive the stellar line-of-sight velocity distribution within one effective radius, R-eff. In addition, the rotation curve and dispersion profile of an ionized gas disk are measured from the [O III] emission lines. After correcting for the asymmetric drift, we derive the circular velocity of the gas, which provides an independent constraint on the gravitational potential. To interpret the stellar motions, we build axisymmetric three-integral dynamical models based on an extension of the Schwarzschild orbit-superposition technique. We consider two families of gravitational potential, one in which the mass follows the light (i.e., no dark matter) and one with a logarithmic gravitational potential. Using chi(2) statistics, we compare our models to both the stellar and gas data to constrain 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 data and possess similar dynamical structure. Across the full range of gamma(V) permitted by the observational constraints, the models are radially anisotropic in the equatorial plane over the radial range of our kinematical data (1 less than or similar to r less than or similar to 40 ). Along the true 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) similar or equal to 0.5-0.6, and sigma(theta)/sigma(total) similar or equal to 0.5 in the equatorial plane.