Radio source contributions to the microwave sky

Cross-correlations of the Wilkinson Microwave Anisotropy Probe (WMAP) full sky K-, Ka-, Q-, V-, and W-band maps with the 1.4 GHz NVSS source count map and the HEAO I A2 2 - 10 keV full sky X-ray flux map are used to constrain rms fluctuations due to unresolved microwave sources in the WMAP frequency range. In the Q band ( 40.7 GHz), a lower limit, taking account of only those fluctuations correlated with the 1.4 GHz radio source counts and X-ray flux, corresponds to an rms Rayleigh-Jeans temperature of similar to 2 mu K for a solid angle of 1 deg(2) assuming that the cross-correlations are dominated by clustering, and similar to 1 mu K if dominated by Poisson fluctuations. The correlated fluctuations at the other bands are consistent with a beta = -2.1 +/- 0.4 frequency spectrum. If microwave sources are distributed similarly in redshift to the radio and X-ray sources and are similarly clustered, then the implied total rms microwave fluctuations correspond to similar to 5 mu K. While this value should be considered no more than a plausible estimate, it is similar to that implied by the excess, small angular scale fluctuations observed in the Q band by WMAP and is consistent with estimates made by extrapolating low-frequency source counts.