Resolving the Shocks in Radio Galaxy Nebulae: Hubble Space Telescope and Radio Imaging of 3C 171, 3C 277.3, and PKS 2250−41^*

We present the results of Hubble Space Telescope (HST) WFPC2 medium-band and narrowband imaging and Very Large Array and MERLIN2 radio imaging of three powerful radio galaxies: 3C 171, 3C 277.3, and PKS 2250-41. We obtained images of the rest frame [O III] λ5007 and [O II] λ3727 line emission using the linear ramp filters on WFPC2. The correlations of the emission-line morphology and the [O III]/[O II] line ratios with the radio emission seen in ground-based observations are clarified by the HST imaging. We confirm that the radio lobes and hot spots are preferentially associated with lower ionization gas. The galaxy 3C 171 exhibits high surface brightness emission-line gas mainly along the radio source axis. The lowest ionization gas is seen at the eastern hot spot. In 3C 277.3 there is bright high-ionization gas (and continuum) offset just to the east of the radio knot K1. Our observations are consistent with previous work suggesting that this emission is produced by precursor gas ionized by the shock being driven into the cloud by the deflected radio jet. In PKS 2250-41 we resolve the emission-line arc that wraps around the outer rim of the western lobe. The lower ionization [O II] emission is nested just interior to the higher ionization [O III] emission, suggesting that we have resolved the cooling region behind the bow shock. We also detect possible continuum emission from the secondary hot spot. Thus, our observations support the hypothesis that in these sources the interaction between the expanding radio source and the ambient gas strongly influences the morphology, kinematics, and ionization of the gas.