Point Source Contamination in CMB Non-Gaussianity Analyses

In this paper we analyze the biasing effect of point sources, either thermal Sunyaev-Zeldovich clusters or standard radio sources, on the estimated strength of the non-Gaussianity in the Cosmic Microwave Background (CMB). We show that the biggest contribution comes from the cross--correlation of the CMB with the matter density rather than from the poisson term which is conventionally assumed in these calculations. For the three year WMAP data, we estimate that point sources could produce a non--Gaussian signature equivalent to a bias in $f_{NL}$ of 0.8 in the Ka band and no greater than 0.2 in the V or W bands. For Planck, we estimate the point source bispectra to contaminate the $f_{NL}$ estimator with a bias of 3 at 30 GHz and 0.6 at 100 GHz. These results significantly depend on the assumed redshift distribution of the point sources. The level of bias we find is largely insufficient to explain the very high f_{NL} values recently detected by Yadav and Wandelt. However, given the projected Planck sensitivity of \Delta f_{NL} \simeq 5 (95 % C.L.), a good estimate of point sources' properties including their number density and redshift distribution is essential before deriving strong conclusions on primordial non--Gaussianity.