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

Hierarchical models of galaxy formation and various observational evidencesuggest that elliptical galaxies are, like disk galaxies, embedded in massivedark matter halos. Stellar kinematics are considered the most important tracerfor this dark halo at a few effective radii. Using detailed modelingtechniques, several authors have recently presented stellar kinematicalevidence of a dark halo for a number of elliptical galaxies. In these modelingtechniques, dust attenuation (absorption and scattering of starlight by dustgrains) has not been taken into account. Nevertheless, elliptical galaxiescontain a significant amount of interstellar dust, which affects all observablequantities, including the observed kinematics. We constructed a set ofdynamical models for elliptical galaxies, in which dust attenuation is includedthrough a Monte Carlo technique. We find that a dust component, shallower thanthe stellar distribution and with an optical depth of order unity, affects theobserved kinematics significantly, in the way that it mimics the presence of adark halo. If such dust distributions are realistic in elliptical galaxies, weare faced with a new mass-dust degeneracy. Taking dust into account indynamical modeling procedures will hence reduce or may even eliminate the needfor a dark matter halo at a few effective radii.Comment: 5 pages, 4 figures, accepted for publication in ApJ Letters. Uses emulateapj5.sty. Typos are correcte