A mass estimate for the companion to the ‘Cartwheel’ galaxy

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Abstract
We have measured the velocity and the velocity dispersion of the elliptical companion to the ring galaxy known as the ‘Cartwheel’. The velocity confirms the suggestion of Fosbury & Hawarden that this companion galaxy was the interloper which created the ring structure. The mass inferred from the dispersion is only 5–10 per cent of the mass of the Cartwheel itself derived from the kinematics of the ring. This low mass is insufficient to account for the observed ring by the collisionless kinematic model of Lynds & Toomre. It seems likely, however, that the underlying mechanism is an encounter of the kind suggested by Lynds & Toomre but that the ring is brightened by the star formation induced.

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