Hubble Space Telescope STIS observations of the kinematics of emission-line nebulae in three compact steep-spectrum radio sources

We have obtained Hubble Space Telescope Space Telescope Imaging Spectrograph long-slit spectroscopy of the aligned emission-line nebulae in three compact steep-spectrum radio sources: 3C 67, 277.1, and 303.1. We find systematic offsets (similar to300-500 km s(-1)) of the emission-line velocities on one or both sides of the radio sources. We also see evidence for broad lines (FWHM similar to 500 km s(-1)) and complex emission-line profiles. In 3C 303.1 the data are consistent with multiple components and possibly split lines. The amplitude of the velocity variations is not so large as to exclude gravitationally induced motions. However, the complex kinematics, the lack of a signature of Keplerian rotation, and the association of the velocity variations with the radio lobes are consistent with the motions being driven by the expansion of the radio source. These kinematic signatures are consistent with an interaction between the expanding radio lobe and the emission-line clouds in which the clouds have been accelerated to velocities similar to300-500 km s(-1). Acceleration of the clouds by the bow shock is plausible given the estimated densities in the clouds and the velocities observed in the much smaller compact symmetric objects and with expansion velocities estimated from spectral aging.