Radio structure and optical kinematics of the cD galaxy Hydra A /3C 218/

A 1400 MHz map of the radio source Hydra A with 50-arcsec resolution is presented which shows that this object is a 3C 31 type source, which is unusual for an object of such high luminosity. Optical velocity measurements indicate that the central, optical galaxy consists of a rapidly rotating core of stars and gas embedded in a nonrotating stellar envelope. There is also evidence for a region of nonrotating higher-excitation gas, redshifted relative to the rotating system by approximately 400 km/s. The position angle of the major axis of the radio core is shown to be aligned with the optical rotation axis. This suggests that the more powerful radio sources have their radio structure more closely aligned with their rotation axes than do the weaker ones; this implies that the mechanism responsible for the alignment of radio and rotation axes is related to the radio power of a source and not its radio morphology.