2004A&A...428..847P

We observed the thirteen extragalactic radio sources of the complete S5 polar cap sample at 15.4GHz with the Very Long Baseline Array, on 27 July 1999 (1999.57) and 15 June 2000 (2000.46). We present the maps from those two epochs, along with maps obtained from observations of the 2cm VLBA survey for some of the sources of the sample, making a total of 40 maps. We discuss the apparent morphological changes displayed by the radio sources between the observing epochs. Our VLBA observations correspond to the first two epochs at 15.4GHz of a program to study the absolute kinematics of the radio source components of the members of the sample, by means of phase delay astrometry at 8.4GHz, 15.4GHz, and 43GHz. Our 15.4GHz VLBA imaging allowed us to disentangle the inner milliarcsecond structure of some of the sources, thus resolving components that appeared blended at 8.4GHz. For most of the sources, we identified the brightest feature in each radio source with the core. These identifications are supported by the spectral index estimates for those brightest features, which are in general flat, or even inverted. Most of the sources display core-dominance in the overall emission, as given by the core-to-extended ratio in the 15.4GHz frame, Q. We find that three of the sources have their most inverted spectrum component shifted with respect to the origin in the map, which approximately coincides with the peak-of-brightness at both 15.4GHz and 8.4GHz.