30 GHz observations of sources in the VSA fields
  Small angular scale (high l) studies of cosmic microwave background
anisotropies require accurate knowledge of the statistical properties of
extragalactic sources at cm-mm wavelengths. We have used a 30 GHz dual-beam
receiver (OCRA-p) on the Torun 32-m telescope to measure the flux densities of
121 sources in VSA fields selected at 15 GHz with the Ryle Telescope. We have
detected 57 sources above a limiting flux density of 5mJy, of which 31 sources
have a flux density greater than 10mJy, which is our effective completeness
limit. From these measurements we derive a surface density of sources above
10mJy at 30 GHz of 2.0+/-0.4 per square degree. This is consistent with the
surface density obtained by Mason et al. (2009) who observed a large sample of
sources selected at a much lower frequency (1.4 GHz). We have also investigated
the dependence of the spectral index distribution on flux density by comparing
our results with those for sources above 1 Jy selected from the WMAP 22 GHz
catalogue. We conclude that the proportion of steep spectrum sources increases
with decreasing flux density, qualitatively consistent with the predictions of
deZotti et al. (2005). We find no evidence for an unexpected population of
sources whose spectra rise towards high frequencies, which would affect our
ability to interpret current high resolution CMB observations at 30 GHz and
above.
