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

Mon. Not. R. Astron. Soc. 422, 3617–3642 (2012) doi:10.1111/j.1365-2966.2012.20875.x Foreground analysis using cross-correlations of external templates on the 7-year Wilkinson Microwave Anisotropy Probe data Tuhin Ghosh,1 A. J. Banday,2,3 Tess Jaffe,2,3 Clive Dickinson,4 Rod Davies,4 Richard Davis4 and Krzysztof Gorski5,6,7 1 IUCAA, Post Bag 4, Ganeshkhind, Pune 411007, India 2 Université de Toulouse, UPS-OMP, IRAP, F-31400 Toulouse, France 3 CNRS, IRAP, 9 Av. colonel Roche, BP 44346, F-31028 Toulouse cedex 4, France 4 Jodrell Bank Centre for Astrophysics, Alan Turing Building, School of Physics & Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL 5 Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA 6 California Institute of Technology, Pasadena, CA 91125, USA 7 Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland 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 ∼6000 K (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 40–50 GHz. 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. Key words: radiation mechanisms: general – cosmology: observations – diffuse radiation – radio continuum: ISM.  E-mail:  C 2012 The Authors C 2012 RAS Monthly Notices of the Royal Astronomical Society  Accepted 2012 March 5. Received 2012 February 15; in original form 2011 December 8 3618 T. Ghosh et al. 1 I N T RO D U C T I O N  C 2012 The Authors, MNRAS 422, 3617–3642 C 2012 RAS Monthly Notices of the Royal Astronomical Society  A major goal of observational cosmology is to determine those parameters that describe the Universe. Observations of the cosmic microwave background (CMB) at frequencies in the range 20–200 GHz provide unique data to achieve this by establishing the statistical properties of temperature (and polarization) measurements. However, an impediment to such studies arises due to foreground emission in our own Galaxy from at least three sources – synchrotron, free–free and thermal dust emission. As CMB studies move to ever higher precision it is essential to determine the properties of these components to similarly high accuracy. Indeed, although the combined foreground level reaches a minimum in this frequency range (ν ≈ 70 GHz), it remains the dominant signal over large fractions of the sky. Of particular relevance to this discussion is the fact that each of the foreground components has a spectral index that varies from one line of sight to another, so using a single spectral index can lead to significant uncertainties in the corrections required. It is therefore essential to study the nature of the Galactic signal at microwave wavelengths in its own right. All-sky observations by the Wilkinson Microwave Anisotropy Probe (WMAP; Bennett et al. 2003a) at the five frequencies of 23, 33, 41, 61 and 94 GHz can provide the basis for improving our understanding of local foregrounds. By comparing these maps with templates for synchrotron, free–free and dust emission, made at frequencies where the specific emission mechanisms dominate, it is possible to clarify important properties of the emission. Indeed, new insights into the nature of the Galactic diffuse emission have arisen from studies of the WMAP data, including the detection of several unexpected new contributions. Anomalous dust-correlated emission (Leitch et al. 1997) was originally observed in the COBE-DMR data (Kogut et al. 1996) but was thought to be due to free–free emission. Draine & Lazarian (1998a,b) shifted attention to the dust itself as the source of emission through a mechanism now referred to as ‘spinning dust’ or dipole emission from very rapidly spinning grains. A reanalysis of the intermediate and high Galactic latitude data taken by COBE-DMR and supplemented by 19-GHz observations (Banday et al. 2003) led to evidence for dust at intermediate Galactic latitudes emitting a spectrum consistent with the form expected for spinning dust, specifically a hint of a turnover at frequencies below ∼20 GHz. However, study of the WMAP data has allowed further refinement of our understanding of the emission. Crosscorrelation of the K-band data with observations at 15 GHz (de Oliveira-Costa et al. 2004) again indicated a plateau or downturn in foreground emission inconsistent with a free–free or synchrotron origin. Lagache (2003) compared the WMAP data to H I column density measurements, revealing an increase in emission with decreasing density, suggesting that the anomalous emission is connected to small, transient heated grains. In addition, a series of papers (Finkbeiner 2004; Dobler & Finkbeiner 2008a,b) have strongly confirmed the (...truncated)