Foregrounds and CMB experiments. I. Semi-analytical estimates of contamination

As Cosmic Microwave Background (CMB) measurements are becoming more ambitious, the issue of foreground contamination is becoming more pressing. This is especially true at the level of sensitivity, angular resolution and for the sky coverage of the planned space experiments ( MAP, 1998) and ( Planck, 1999). We present in this paper an indicator of the accuracy of the separation of the CMB anisotropies from those induced by foregrounds. Of course, the outcome will depend on the spectral and spatial characteristics of the sources of anisotropies. We thus start by summarising the present knowledge on the spectral and spatial properties of Galactic foregrounds, point sources, and clusters of galaxies. This information comes in support of a modelling of the microwave sky including the relevant components. The accuracy indicator we introduce is based on a generalisation of the Wiener filtering method to multi-frequency, multi-resolution data. While the development and use of this indicator was prompted by the preparation of the scientific case for the ( Planck, 1999) satellite, it has broader application since it allows assessing the effective capabilities of an instrumental set-up once foregrounds are fully accounted for, with a view to enabling comparisons between different experimental arrangements. The real sky might well be different from the one assumed here, and the analysis method might not be in the end Wiener filtering, but this work still allow meaningful comparative studies. As a matter of examples, we compare the CMB reconstruction errors for the ( MAP, 1998) and ( Planck, 1999) space missions, as well as the robustness of the ( Planck, 1999) outcome to possible failures of specific spectral channels or global variations of the detectors noise level across spectral channels.