We discuss Hubble Space T elescope (HST ) WFPC2 observations through the broad red Ðlter F702W of 30 3CR sources from the Compact Steep Spectrum (CSS) radio source sample, and present 11 new HST /WFPC2 images through linear ramp Ðlters (LRF), isolating either rest-frame [O II] 3727 or A [O III] 5007 radiation. In nearly all the CSS galaxies, we Ðnd high surface brightness emission that is A aligned with the radio axis. The strong nuclear PSF prevents us from detecting such aligned light at similar levels in most of the CSS quasars. However, a comparison between CSS galaxies and quasars with PSF signatures removed reveals no inconsistency with the viewing angle uniÐcation scheme. The alignment e †ect in CSS sources is not a strong function of redshift, and is seen over the entire redshift range of the sample Our analysis of the LRF images and complementary KPNO spec-(0.2 [ z [ 1.5). troscopy reveals that the aligned light is predominantly emission-line gas. These observations demonstrate the existence of dense gas in the host galaxies of CSS sources, strongly interacting with the expanding radio sources. Assuming such a shock interaction, cooling-time arguments suggest that lobe expansion speeds of km s~1 and ambient densities of D1 cm~3 are consistent with the observed Z1000 o †set between the leading edge of the radio lobe and the optical line emission.

We present near-infrared J and K band imaging of a sample of powerful radio source host galaxies with the Hubble Space Telescope NICMOS2 camera. These sources have been selected on their double lobed radio structure, and include a wide range of projected radio source sizes. The largest projected linear sizes range from the compact Gigahertz Peaked Spectrum (GPS, < 1 kpc) and Compact Steep Spectrum (CSS, < 20 kpc) radio sources, up to the large-scale (> 20 kpc) classical doubles (FR II radio sources). We investigate the dependence of host galaxy properties (including near-IR surface brightness profiles) on radio source size, using both our own and published data. The absolute magnitudes and surface brightness profiles are consistent with the host galaxies being regular giant elliptical galaxies rather than Brightest Cluster Galaxies (BCGs). We find that the GPS, CSS, and FR II host galaxies are a uniform class of objects, consistent with a scenario in which a powerful radio source evolves along this size sequence.