Galactic Free-free Emission and Hα

We review present understanding of Galactic free–free emission and its possible importance to CMB fluctuation measurements. Current results, from both " direct " observations in the microwave band and from Hα studies, suggest that this foreground does not represent a serious obstacle to mapping the CMB; however, this is based on limited information and we emphasize the need for more exhaustive studies. We also present some preliminary results based on our recent Hα observations near the South Pole CMB data sets. The fluctuation amplitude seen in Hα indicates that the detected CMB fluctuations are not significantly contaminated by free–free emission, at least if the diffuse gas is at a temperature of T ∼ 10 4 K. The three main sources of microwave emission from our Galaxy are synchrotron radiation, thermal dust emission and bremsstrahlung (free-free emission). Among these three, bremsstrahlung poses the greatest difficulty for cosmic microwave background (CMB) observations for two reasons: 1/ it dominates over the frequency range where the total Galactic emission is minimal, and hence where the majority of CMB experiments are performed, and 2/ it dominates only over a small range of frequencies (see Fig. 3 of Kogut et al. 1996a). Thus, it is the principal Galactic contaminant of many CMB experiments, but it cannot be traced by observing at either much