Reappraising foreground contamination in the COBE-DMR data

With the advent of all-sky H-Alpha surveys it is possible to determine a reliable free-free template of the diffuse interstellar medium (Dickinson, Davies & Davis 2003) which can be used in conjunction with the synchrotron and dust templates to correct CMB observations for diffuse Galactic foregrounds. We have used the COBE-DMR data at 31.5, 53 and 90 GHz and employed cross- correlation techniques to re-evaluate the foreground contributions, particularly that due to dust which is known to be correlated with H-Alpha (and free-free) emission. The DMR microwave maps are found to contain, as well as the expected synchrotron and free-free components, a component tightly correlated to the 140 micron dust maps of DIRBE. At 31.5, 53 and 90 GHz this emission is 6.3 +/- 0.6, 2.4 +/- 0.4 and 2.2 +/- 0.4 microK/(MJy/sr) at 140 microns respectively. When corrected for the contribution from thermal dust following model 7 of Finkbeiner, Davis & Schlegel (1999), a strong anomalous dust emission component remains, which is well-fitted by a frequency spectrum of the form $\nu^{-\beta}$ where beta ~ 2.5; this is the dominant foreground at 31.5 GHz. When lower frequency data are combined with those from the DMR, a spectrum emerges similar in shape to that predicted by Draine & Lazarian (1998a,b). The better knowledge of the individual foregrounds provided by the present study enables a larger area of the sky (|b|>15 degrees) to be used to re-appraise the CMB quadrupole normalisation, Qrms-PS, and the scalar perturbations spectral index, n. We find Qrms-PS=$16.0^{+3.0}_{-2.4}$ with a power-law spectral index of $n=1.23^{+0.22}_{-0.23}$. These values are consistent with, and more accurate, than those derived by the COBE team (Bennett et al. 1996).