Foreground analysis using cross-correlations of external templates on the 7-year Wilkinson Microwave Anisotropy Probe data

Abstract

Wilkinson Microwave Anisotropy Probe (WMAP) data when combined with ancillary data on free-free, synchrotron and dust allow an improved understanding of the spectrum of emission from each of these components. Here we examine the sky variation at intermediate and high latitudes using a cross-correlation technique. In particular, we compare the observed emission in several global partitions of the sky plus 33 selected sky regions to three 'standard' templates. The regions are selected using a criterion based on the morphology of these template maps. The synchrotron emission shows evidence of steepening between GHz frequencies and the WMAP bands. There are indications of spectral index variations across the sky, but the current data are not precise enough to accurately quantify this from region to region. The Hα template correlated emission derived from the global fits shows clear evidence of deviation from a free-free spectrum. If this spectrum is decomposed into a contribution from both free-free and spinning dust emission in the warm ionized medium of the Galaxy, the derived free-free emissivity corresponds to a mean electron temperature of ∼6000K (a value critically dependent on the impact of dust absorption on the Hα intensity), and the spinning dust emission has a peak emission in intensity typically in the range 40-50GHz. However, the analysis of the smaller regions is generally unrevealing and the analysis presented here does not unambiguously demonstrate the presence of spinning dust emission in the warm ionized medium, as advocated by Dobler & Finkbeiner. The anomalous microwave emission associated with dust is detected at high significance in most of the 33 fields studied. The anomalous emission correlates well with the Finkbeiner et al. model 8 predictions (FDS8) at 94 GHz, and is well described globally by a power-law emission model with an effective spectral index between 20 and 60 GHz of β≈-2.7. It is clear that attempts to explain the emission by spinning dust models require multiple components, which presumably relates to a complex mix of emission regions along a given line of sight. An enhancement of the thermal dust contribution over the FDS8 predictions by a factor ∼1.2 is required with such models. Furthermore, the emissivity varies by a factor of ∼50 per cent from cloud to cloud relative to the mean. The significance of these results for the correction of cosmic microwave background data for Galactic foreground emission is discussed. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.