The radio source counts at 15 GHz and their implications for cm-wave CMB imaging

We use the preliminary results of a new survey of radio sources made using the Ryle Telescope at 15.2GHz, to estimate the impact of foreground sources on cm-wave cosmic microwave background (CMB) images. This is the highest frequency survey that is relevant to the issue of radio source contamination in CMB experiments. The differential source count of the 66 sources found in 63deg2 is 80(S/Jy)-2.0Jy-1sr-1, from ≈20 to ≈500 mJy. Extrapolating this to 34 GHz (where many cm-wave CMB experiments operate) gives an estimated temperature contribution of sources ΔTconf = 9 μK in a CMB image, with a beam corresponding to multipole l ≈ 500. A means of source subtraction is evidently necessary, otherwise the signal-to-noise ratio in CMB images will be limited to 4 or 5, becoming worse at higher resolution. We compare the population of sources observed in this new survey to that predicted by extrapolation from lower frequency surveys, finding that source flux densities, and indeed the existence of many sources, cannot be determined by extrapolation.