Radio Sources toward Galaxy Clusters at 30 GHz

Extra-galactic radio sources are a significant contaminant in cosmicmicrowave background and Sunyaev-Zel'dovich effect experiments. Deepinterferometric observations with the BIMA and OVRO arrays are used tocharacterize the spatial, spectral, and flux distributions of radio sourcestoward massive galaxy clusters at 28.5 GHz. We compute counts of mJy sourcefluxes from 89 fields centered on known massive galaxy clusters and 8non-cluster fields. We find that source counts in the inner regions of thecluster fields (within 0.5 arcmin of the cluster center) are a factor of 8.9(+4.3,-2.8) times higher than counts in the outer regions of the cluster fields(radius greater than 0.5 arcmin). Counts in the outer regions of the clusterfields are in turn a factor of 3.3 (+4.1,-1.8) greater than those in thenon-cluster fields. Counts in the non-cluster fields are consistent withextrapolations from the results of other surveys. We compute spectral indicesof mJy sources in cluster fields between 1.4 and 28.5 GHz and find a meanspectral index of alpha = 0.66 with an rms dispersion of 0.36, where flux isproportional to frequency raised to negative alpha. The distribution is skewed,with a median spectral index of 0.72 and 25th and 75th percentiles of 0.51 and0.92, respectively. This is steeper than the spectral indices of stronger fieldsources measured by other surveys.Comment: 32 pages, 6 figures, accepted to A