Dark Matter Halos around Elliptical Galaxies: How Reliable Is the Stellar Kinematical Evidence?

Abstract

Hierarchical models of galaxy formation and various observational evidence suggest that elliptical galaxies are, like disk galaxies, embedded in massive dark matter halos. Stellar kinematics are considered the most important tracer for this dark halo at a few effective radii. Using detailed modeling techniques, several authors have recently presented stellar kinematical evidence of a dark halo for a number of elliptical galaxies. In these modeling techniques, dust attenuation (absorption and scattering of starlight by dust grains) has not been taken into account. Nevertheless, elliptical galaxies contain a significant amount of interstellar dust, which affects all observable quantities, including the observed kinematics. We constructed a set of dynamical models for elliptical galaxies, in which dust attenuation is included through a Monte Carlo technique. We find that a dust component, shallower than the stellar distribution and with an optical depth of order unity, affects the observed kinematics significantly, in the way that it mimics the presence of a dark halo. If such dust distributions are realistic in elliptical galaxies, we are faced with a new mass-dust degeneracy. Taking dust attenuation into account in dynamical modeling procedures will hence reduce or may even eliminate the need for a dark matter halo at a few effective radii.