Jet-Cloud Interactions in Compact Steep Spectrum Radio Sources

We have obtained HST/STIS long slit spectroscopy of the aligned emission line nebulae in three compact steep spectrum (CSS) radio sources - 3C67, 3C277.1, and 3C303.1. We find systematic offsets (~300-500kms) of the emission line velocities on one or both sides of the radio sources. We also see evidence for broad lines (FWHM~500kms) and complex emission line profiles. In 3C303.1 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 observed ~300-500kms velocities being driven by the expansion of the radio source. 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 ageing. This conclusion is unchanged if we consider the scenario in which the cloud acceleration is dominated by the post bow shock flow.