A 31 GHz Survey of Low-Frequency Selected Radio Sources

The 100 m Robert C. Byrd Green Bank Telescope and the 40 m Owens Valley Radio Observatory telescope have been used to conduct a 31 GHz survey of 3165 known extragalactic radio sources over 143 deg^2 of the sky. Target sources were selected from the NRAO VLA Sky Survey in fields observed by the Cosmic Background Imager (CBI); most are extragalactic active galactic nuclei (AGNs) with 1.4 GHz flux densities of 3-10 mJy. The resulting 31 GHz catalogs are presented in full online. Using a maximum-likelihood analysis to obtain an unbiased estimate of the distribution of the 1.4-31 GHz spectral indices of these sources, we find a mean 31-1.4 GHz flux ratio of 0.110 ± 0.003 corresponding to a spectral index of α = –0.71 ± 0.01 (S_ν ∝ ν^α); 9.0% ± 0.8% of sources have α > –0.5 and 1.2% ± 0.2% have α > 0. By combining this spectral-index distribution with 1.4 GHz source counts, we predict 31 GHz source counts in the range 1 mJy < S_(31) < 4 mJy, N(>S_(31)) = (16.7 ± 1.7) deg^(-2)(S_(31)/1 mJy)^(–0.80±0.07). We also assess the contribution of mJy-level (S_(1.4 GHz) < 3.4 mJy) radio sources to the 31 GHz cosmic microwave background power spectrum, finding a mean power of ℓ(ℓ + 1)C^(src)_ℓ /(2π) = 44 ± 14 μK^2 and a 95% upper limit of 80 μK^2 at ℓ = 2500. Including an estimated contribution of 12 μK^2 from the population of sources responsible for the turn-up in counts below S_(1.4 GHz) = 1 mJy, this amounts to 21% ± 7% of what is needed to explain the CBI high-ℓ excess signal, 275 ± 63 μK^2. These results are consistent with other measurements of the 31 GHz point-source foreground.