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 at 2.3 and 8.6 GHz 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. 34% of the sources show parsec-scale
emission above the flux density 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 72%,
has a steep single-dish 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 54% of the sources detected by the VLBA at
2.3 GHz in our sample have a steep VLBA spectrum. The compact radio emission of
these sources is likely dominated by optically thin jets or mini-lobes, not by
an opaque jet core. These results show that future VLBI surveys aimed to search
for new sources with parsec-scale structure should include not only
flat-spectrum sources, but also steep-spectrum ones in order to reach an
acceptable level of completeness.
