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We present line-of-sight stellar velocity distributions of elliptical galaxyNGC 821 obtained to approximately 100 arcsec (over 2 effective radii) withlong-slit spectroscopy from the Hobby-Eberly Telescope. Our measured stellarline-of-sight velocity distributions are larger than the planetary nebulaemeasurements at similar radii. We fit axisymmetric orbit-superposition modelswith a range of dark halo density profiles, including two-dimensionalkinematics at smaller radii from SAURON data. Within our assumptions, thebest-fitted model gives a total enclosed mass of 2.0x10^11 M_sun within 100arcsec, with an accuracy of 2%; this mass is equally divided between halo andstars. At 1R_e the best-fitted dark matter halo accounts for 13% of the totalmass in the galaxy. This dark halo is inconsistent with previous claims oflittle to no dark matter halo in this galaxy from planetary nebulameasurements. We find that a power-law dark halo with a slope 0.1 is thebest-fitted model; both the no dark halo and NFW models are worse fits at agreater than 99% confidence level. NGC 821 does not appear to have the expecteddark halo density profile. The internal moments of the stellar velocitydistribution show that the model with no dark halo is radially anisotropic atsmall radii and tangentially isotropic at large radii, while the best-fittedhalo models are slightly radially anisotropic at all radii. We test thepotential effects of model smoothing and find that there are no effects on ourresults within the errors. Finally, we run models using the planetary nebulakinematics and assuming our best-fitted halos and find that the planetarynebulae require radial orbits throughout the galaxy.

Hobby-Eberly Telescope Observations of the Dark Halo in NGC 821