We present an analysis of multi-epoch global VLBI observations of the Compact Symmetric Objects: 2352+495 and 0710+439 at 5 GHz. Analysis of data spread over almost two decades shows strong evidence for an increase in separation of the outer components of both sources at a rate of ∼ 0.2h −1 c (for q • =0.5 and H • = 100h kms −1 Mpc −1 ). Dividing the overall sizes of the sources by their separation rates implies that these Compact Symmetric Objects have a kinematic age ≪ 10 4 years. These results (and those for other CSOs) strongly argue that CSOs are indeed very young sources and that they are probably evolve into the much larger classical doubles.

Abstract.We consider the incidence of H absorption in intrinsically small sub-galactic sized extragalactic sources selected from sources classified as Gigahertz Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) sources. We find that the smaller sources (<0.5 kpc) have larger H column densities than the larger sources (>0.5 kpc). Both a spherical and an axisymmetric gas distribution, with a radial power law density profile, can be used to explain this anti-correlation between projected linear size and H column density. Since most detections occur in objects classified as galaxies, we argue that if the unified schemes apply to GPS/CSSs a disk distribution for the H is more likely. The most favoured explanation for the compact sizes of the GPS/CSSs is that they are young sources evolving in a power law density medium. For the GPSs with measured expansion velocities, our derived densities are within an order of magnitude of those estimated from ram-pressure confinement of the lobes assuming equipartition. Our results therefore support the youth model.

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We present results of VLA and VLBA observations of the 1.420 GHz neutral hydrogen absorption associated with the Compact Symmetric Object 1946+708 (z=0.101). We find significant structure in the gas on parsec scales. The peak column density in the Hi (N HI ≃2.2×10 23 cm −2 ( Ts 8000K ) ) occurs toward the center of activity of the source, as does the highest velocity dispersion (∆V FWHM ≃350 km s −1 ). In addition, we find that the continuum spectra of the various radio components associated with these jets strongly indicate free-free absorption. This effect is particularly pronounced toward the core and inner components of the receding jet, suggesting the presence of a screen local to the source, perhaps part of an obscuring torus.

EVN and MERLIN observations of the 18 cm continuum and OH maser emission in the luminous infrared galaxy III Zw 35 are presented. The continuum consists of two components. The first component is made up of a number of compact sources, resembling the types of objects observed in Arp 220 and Mrk 273. In III Zw 35 the origin of these compact components is unclear, and we discuss the possibility they are individual young radio supernovae. Alternatively they could be clusters or clumps of young supernova remnants. The second continuum component is diffuse and has an angular size of ∼460 mas (230 pc) and T b ∼ 10 5 K. We argue that this component is also powered by star formation. From the F IR flux we estimate a star formation rate of 19 M yr −1 , which in turn implies a supernova rate of 0.8 yr −1 , sufficient to power the diffuse radio emission. Turning to the OH maser emission we find, in agreement with earlier VLBI observations of III Zw 35, that the bright OH maser emission is concentrated in two compact clumps. However, in addition we observe diffuse maser emission which lies in a clear ring structure of radius r ∼ 22 pc. The two compact maser features occur at the tangent points of the ring. This geometry and their similar 1667/1665 MHz line ratios suggests that the compact and diffuse masers appear different because of path length effects and not pumping or other physical conditions. A simple model with many clouds within a ring seems able to fit the main properties of both the diffuse and compact maser emission. We find a velocity gradient across the diffuse maser emitting region which confirms rotation. From this gradient we estimate a dynamical mass of 7 × 10 6 M within r ∼ 22 pc. OH absorption is also present just outside the ring of maser emission. Both the OH maser emission and the continuum is weaker on the Eastern side of the source. We argue that this could be due to free-free absorption effects, which implies gas with an emission measure of 7 × 10 6 cm −6 pc.