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 15.2 GHz, to estimate the impact of foreground source s on cin-wave cosmic microwave background (CMB) images. This is the highest frequency survey that is relevant to the issue of radio source contaminati on in CMB experiments. The differential source count of the 66 sources foun d in 63 deg(2) is 80(S/Jy)(-2.0) Jy(-1) sr(-1), from approximate to 20 to a pproximate to 500 mJy. Extrapolating this to 34 GHz (where many cm-wave CMB experiments operate) gives an estimated temperature contribution of source s DeltaT(conf) = 9 muK in a CMB image, with a beam corresponding to multipo le l approximate to 500. A means of source subtraction is evidently necessa ry, 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 sou rces observed in this new survey to that predicted by extrapolation from lo wer frequency surveys, finding that source flux densities, and indeed the e xistence of many sources, cannot be determined by extrapolation.