Radio Source Counts at 15 GHz: Implications for CMB Experiments

In preparation for Very Small Array (VSA) observations we have surveyed the first three VSA fields at 15 GHz using the Ryle Telescope in Cambridge, finding 217 sources with flux density greater than ˜ 20 mJy over 178 deg 2 . This is the highest radio frequency at which a survey has been done that is relevant to the issue of radio source contamination in CMB experiments. Our results show that source fluxes, and indeed the existence of many sources, cannot be predicted at 15 GHz by extrapolation from lower frequency. The spectra and variability of the sources and their implications for CMB experiments at cm-wavelengths are discussed.