Milli-arcsecond properties of 10C sources in the Lockman Hole
  We have used recent Very Long Baseline Interferometry (VLBI) observations by
Middelberg et al. with a resolution of $\approx 10$ mas to investigate the
properties of faint sources selected from the Tenth Cambridge (10C) survey in
the Lockman Hole. The 10C survey is complete to 0.5 mJy at 15.7 GHz and has a
resolution of 30 arcsec. We have previously shown that this population is
dominated by flat-spectrum sources below $\approx 1$ mJy, in disagreement with
several models of the faint, high-frequency sky. We find that 33 out of the 51
10C sources in the VLBI field (65 percent) are detected by the VLBI
observations. The sources detected by the VLBI observations must have a high
brightness temperature, thus ruling out the possibility that this faint, high
frequency population is dominated by starbursting or starforming sources and
indicating that they must be Active Galactic Nuclei.
