Halo Kinematics

Abstract

I summarize recent observations of the kinematics of hot tracers in elliptical galaxy halos (globular clusters, planetary nebulae, and integrated stellar light), and what these tell us about the dynamics, dark matter content, and formation of ellipticals. A generic result is the ubiquity of dark matter halos in ellipticals. Studies of globular clusters and planetary nebulae are now finding outer-halo rotation in many ellipticals, with V/sigma ~ 1 beyond a few R_e. In some giant ellipticals (M49, M87), there are possible kinematic differences between metal-poor and metal-rich globular clusters. These results are consistent with a merger origin for ellipticals. High-quality data and new modelling techniques now make it possible to determine simultaneously the orbital anisotropy and gravitational potential in ellipticals from integrated-light measurements; such studies now provide the best evidence for dark matter halos in ellipticals.The new generation of 8-10m telescopes, with multi-object and integral-field spectrographs, will dramatically increase sample sizes of discrete tracers and provide two-dimensional spectroscopy of elliptical halos. New methods of analysis will allow robust determinations of stellar kinematics and dark matter distributions in a much larger number of ellipticals. Comparison with numerical simulations, which are becoming ever more detailed and physically realistic, will become increasingly important.