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Abstract. We present VLBI observations at 5 GHz of 11 GHz Peaked Spectrum (GPS) or candidate GPS sources. Two of them belong to the complete sample defined by Stanghellini et al. (1998) (the GPS 1 Jy sample), while the others are selected from a heterogeneous list compiled by O'Dea et al. (1991). Morphologies of the sources presented here are similar to those found in Stanghellini et al. (1997) and in the small Compact Steep Spectrum (CSS) sources studied by Dallacasa et al. (1995).Our results strengthen the evidence that GPS quasars tend to have core-jet or complex morphology and GPS galaxies tend to be compact symmetric objects (CSO).

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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.