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

The best constraints on dark halo properties of elliptical galaxies come from large radii; however, observations at these radii are difficult due to the faint stellar light and/or sparse kinematic tracers (i.e., globular clusters, planetary nebulae). We present line-of-sight stellar velocity distributions of elliptical galaxy NGC 821 obtained to approximately 100 (over 2 effective radii) with long-slit spectroscopy from the Hobby-Eberly Telescope. We fit axisymmetric orbit-superposition models with a range of dark halo density profiles including SAURON data and find that a power-law dark halo with a slope 0.1 and central density 0.025M⊙/pc 3 is the best-fitted model; both the no dark halo and NFW models are worse fits at a greater than 99% confidence level. NGC 821 does not appear to have the expected dark halo shape. The best-fitted model gives a total enclosed mass of 2.0 × 10M⊙ within 100, equally divided between halo and stars. At 1Re the best-fitted dark matter halo accounts for 13% of the total mass in the galaxy. The internal moments of the stellar velocity distribution show that the model with no dark halo is radially anisotropic at small radii and tangentially isotropic at large radii, while the best-fitted halo models are slightly radially anisotropic at all radii. Finally, we test the potential effects of model smoothing and find that there are no effects on our results within the errors. The dark halo we find is inconsistent with previous claims of little to no dark matter halo in this galaxy from planetary nebula measurements. Additionally, our measured stellar line-of-sight stellar velocity distributions are larger than the planetary nebulae measurements at large radii. We run models using the planetary nebula kinematics assuming our best-fitted halos and find that the planetary nebulae require radial orbits throughout the galaxy. Subject headings: galaxies: elliptical and lenticular, cD — galaxies: individual(NGC 821) — galaxies: kinematics and dynamics — galaxies: halos — stellar dynamics