SEPARATION OF FOREGROUND RADIATION FROM COSMIC MICROWAVE BACKGROUND ANISOTROPY USING MULTIFREQUENCY MEASUREMENTS

Future measurements of the isotropy of the cosmic microwave background radiation (CBR) will be limited by confusing foreground radiation. We examine the extent to which galactic foregrounds-synchrotron, bremsstrahlung, and dust radiation-can be identified and removed using multifrequency data. We discuss the capabilities and limitations of this approach and investigate its effectiveness for various choices of observing frequencies using simulations over 10-degrees x 10-degrees patches of sky with 1-degree resolution. At this resolution galactic fluctuations are at a minimum just below 100 GHz. In our simulations, observations covering two or three octaves in frequency well below or well above this minimum region perform much better than those in the minimum region and allow accurate subtraction of foregrounds if the measurement noise is sufficiently low. We are thus optimistic that Galactic foreground fluctuations can be distinguished from CBR fluctuations in multifrequency data on 1-degree scales. The required noise level depends in a highly nonlinear way on the frequencies observed, the model used to fit the various signal components, and on (unknown) details of the foreground emissions and CBR anisotropy. Even in favorable cases, however, the required noise level is much below any achieved to date. Separation of the CBR from foregrounds required at least three free parameters in the simulations, thus observations at a minimum of four frequencies are required for our method. We also discuss the usefulness of measurements of foreground components made at frequencies where those foregrounds are strong, and we show that even if the functional form of the spectrum of the foregrounds is perfectly known, the absolute error that can be tolerated in measurements at other frequencies is comparable to the error that can be tolerated where the CBR is measured. Thus there is no easy way to subtract confusing foregrounds directly.