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

Understand

We present line-of-sight stellar velocity distributions of elliptical galaxy NGC 821 obtained to approximately 100 arcsec (over 2 effective radii) with long-slit spectroscopy from the Hobby-Eberly Telescope.

• Our measured stellar line-of-sight velocity distributions are larger than the planetary nebulae measurements at similar radii.
• We fit axisymmetric orbit-superposition models with a range of dark halo density profiles, including two-dimensional kinematics at smaller radii from SAURON data.
• Within our assumptions, the best-fitted model gives a total enclosed mass of 2.0x10^11 M_sun within 100 arcsec, with an accuracy of 2%; this mass is equally divided between halo and stars.