Kinematics and dynamics of the haloes of supergiant galaxies

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Abstract
We present measurements of the internal kinematics of the haloes of three supergiant (cD or giant dumb-bell) galaxies in moderately rich clusters. We find that these galaxies have velocity dispersion profiles which are flat or which rise with increasing radius. Our observations can equally well be explained by two simple models. The first class has isotropic velocity dispersions and a two-component mass distribution: a bright component with a density distribution which follows a King model, and a dark, isothermal component with a density distribution which is slightly more concentrated than that of the galaxies in the cluster as a whole. The second class has predominantly tangential motions in the halo of the galaxy, and no dark component. In common with most luminous elliptical galaxies these supergiant galaxies are not flattened by rotation. In the two dumb-bells studied the relative velocities between the nuclei are comparable to the stellar velocity dispersions in the cD envelopes.

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