Noncosmological signal contributions to the COBE DMR 4 year sky maps

We limit the possible contributions from noncosmological sources to the COBE Differential Microwave Radiometer (DMR) 4 year sky maps. The DMR data are cross-correlated with maps of rich clusters, extragalactic IRAS sources, HEAO 1 A-2 X-ray emission, and 5 GHz radio sources using a Fourier space technique. There is no evidence of significant contamination by such sources at an rms level of ∼8 μK [95% confidence level (c.l.) at 7° resolution] in the most sensitive 53 GHz sky map. This level is consistent with previous limits set by analysis of earlier DMR data and by simple extrapolations from existing source models. We place a limit on the rms Comptonization parameter averaged over the high-latitude sky of δγ < 1 × 10^-6 (95% c.l.). Extragalactic sources have an insignificant effect on the cosmic microwave background power spectrum parameterizations determined from the DMR data.