Reappraising foreground contamination in the COBE-DMR data

Abstract

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 $ν^{-β}$ where beta ~ 2.5 in the DMR frequency range; this is the dominant foreground at 31.5 GHz. The result implies the presence of an emission component with a dust-like morphology but a synchrotron-like spectrum. We discuss the possible origins of this component and compare it with the recent WMAP interpretation. The better knowledge of the individual foregrounds provided by the present study enables a larger area of the sky (|b|>15) to be used to re-appraise the CMB quadrupole normalisation, Qrms-PS, and the scalar perturbations spectral index, n. We find Qrms-PS=15.2^{+2.8}_{-2.3} with a power-law spectral index of n=1.2 +/- 0.2. These values are consistent with previous COBE-DMR analyses and the WMAP 1-year analysis.