Radio Sources in Galaxy Clusters at 28.5 GHz

We present serendipitous detections of radio sources at 28.5 GHz (1 cm), which resulted from our program to image the thermal Sunyaev-Zeldovich (SZ) effect in 56 galaxy clusters. In a total area of ∼0.8 deg2, we find 64 radio sources with fluxes down to ∼0.4 mJy (greater than 4 σ) and within 250″ from the pointing centers. The spectral indices (S ∝ ν-α) of 54 sources with published low-frequency flux densities lie in the range -0.6 ≲ α ≲ 2, with a mean of 0.77 ± 0.06 and a median of 0.84. Extending low-frequency surveys of radio sources toward the galaxy clusters Cl 0016+16, Abell 665, and Abell 2218 to 28.5 GHz and selecting sources with S1.4 GHz ≥ 7 mJy to form an unbiased sample, we find a mean spectral index of 0.71 ± 0.08 and a median of 0.71. We find 4 to 7 times more sources than predicted from a low-frequency survey in areas without galaxy clusters. This excess cannot be accounted for by gravitational lensing of a background radio population by cluster potentials, indicating that most of the detected sources are associated with galaxy clusters. The differential source count slope, γ ∼ 1.96 (dN/dS ∝ S-γ), is flatter than what is expected for a nonevolving Euclidean population (γ = 2.5). For the cluster Abell 2218, the presence of unsubtracted radio sources with S28.5 GHz ≤ 0.5 mJy (∼5 σ) can reduce the observed SZ temperature decrement by ΔT ∼ 10 to 25 μK. The corresponding overestimation of the Hubble constant derived through a combined analysis of 28.5 GHz SZ images and X-ray emission observations is less than 6%.