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

TL;DR

This study uses Hobby-Eberly Telescope data to analyze the dark matter halo in galaxy NGC 821, finding a significant dark halo inconsistent with previous planetary nebulae measurements, and revealing complex stellar velocity anisotropies.

Contribution

It provides the first detailed dynamical modeling of NGC 821's dark halo using integrated stellar kinematics, challenging prior claims of negligible dark matter in this galaxy.

Findings

Total mass within 100 arcsec is 2.0x10^11 M_sun with 2% accuracy.

Dark halo accounts for 13% of mass at 1 effective radius.

Power-law dark halo with slope 0.1 fits best, rejecting no halo and NFW models.

Abstract

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. At 1R_e the best-fitted dark matter halo accounts for 13% of the total mass in the galaxy. This dark halo is inconsistent with previous claims of little to no dark matter halo in this galaxy from planetary nebula measurements. We find that a power-law dark halo with a slope 0.1 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 density profile. 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. We test the potential effects of model smoothing and find that there are no effects on our results within the errors. Finally, we run models using the planetary nebula kinematics and assuming our best-fitted halos and find that the planetary nebulae require radial orbits throughout the galaxy.