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We extend and apply a model-independent analysis method developed earlier by Daly & Djorgovski to new samples of supernova standard candles, radio galaxy and cluster standard rulers, and use it to constrain physical properties of the dark energy as functions of redshift. Similar results are obtained for the radio galaxy and supernova data sets, which rely upon completely independent methods, suggesting that systematic errors are relatively small for both types of distances; distances to SZ clusters show a scatter which cannot be explained by the quoted measurement errors. The first and second derivatives of the distance are compared directly with predictions in a standard model based on General Relativity. The good agreement indicates that General Relativity provides an accurate description of the data on look-back time scales of about ten billion years. The first and second derivatives are combined to obtain the acceleration parameter q(z), assuming only the validity of the Robertson-Walker metric, independent of a theory of gravity and of the physical nature of the dark energy. The data are analyzed using a sliding window fit and using fits in independent

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).

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.

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