Parsec-scale Properties of Steep- and Flat-spectrum Extragalactic Radio Sources from a VLBA Survey of a Complete North Polar Cap Sample

We observed with the VLBA a complete flux-density limited sample of 482 radio sources with declination >+75 degrees brighter than 200 mJy at 1.4 GHz drawn from the NVSS catalog. One-third of the sources show parsec-scale emission above the detection limit of 30 mJy; their accurate positions and parsec-scale structure parameters are determined. Among all the sources detected at least at the shortest VLBA baselines, the majority, or 70%, has a steep spectrum. The fraction of the sources with a detectable parsec-scale structure is above 95% among the flat-spectrum and close to 25% among the steep-spectrum objects. We identified 82 compact steep-spectrum source candidates, which make up 17% of the sample; most of them are reported for the first time. The compactness and the brightness temperature of the sources in our sample show a positive correlation with single-dish and VLBA spectral indices. All the sources with a significant 8 GHz variability were detected by the VLBA snapshot observations, which independently confirmed their compactness. We demonstrated that the radio emission at 2 GHz comes from optically thin outer parts of jets or from mini-lobes in about a half of compact extragalactic sources drawn from a 1.4 GHz flux-density limited sample. These results should be taken into account while constructing source samples for future VLBI surveys aimed to search for new compact sources.