Component separation methods for the PLANCK mission

Abstract

Context. The planck satellite will map the full sky at nine frequencies from 30 to 857 GHz. The CMB intensity and polarization that are its prime targets are contaminated by foreground emission. Aims. The goal of this paper is to compare proposed methods for separating CMB from foregrounds based on their different spectral and spatial characteristics, and to separate the foregrounds into "components" with different physical origins (Galactic synchrotron, free-free and dust emissions; extra-galactic and far-IR point sources; Sunyaev-Zeldovich effect, etc.). Methods. A component separation challenge has been organised, based on a set of realistically complex simulations of sky emission. Several methods including those based on internal template subtraction, maximum entropy method, parametric method, spatial and harmonic cross correlation methods, and independent component analysis have been tested. Results. Different methods proved to be effective in cleaning the CMB maps of foreground contamination, in reconstructing maps of diffuse Galactic emissions, and in detecting point sources and thermal Sunyaev-Zeldovich signals. The power spectrum of the residuals is, on the largest scales, four orders of magnitude lower than the input Galaxy power spectrum at the foreground minimum. The CMB power spectrum was accurately recovered up to the sixth acoustic peak. The point source detection limit reaches 100 mJy, and about 2300 clusters are detected via the thermal SZ effect on two thirds of the sky. We have found that no single method performs best for all scientific objectives.Conclusions. We foresee that the final component separation pipeline for planck will involve a combination of methods and iterations between processing steps targeted at different objectives such as diffuse component separation, spectral estimation, and compact source extraction.

Planckintermediate results

“…The needlet decomposition provides localisation of the ILC filters both in pixel and in multipole space, allowing us to deal with local contamination conditions varying both in position and in scale. NILC was developed to extract a CMB map from WMAP data (Delabrouille et al 2009) and was also tested for SZ effect extraction in Leach et al (2008). Multi-component extensions have been investigated by Remazeilles et al (2011).…”

A comparison of algorithms for the construction of SZ cluster catalogues

“…The CMB realisation is not constrained by actual observed CMB multipoles, in contrast to the simulations used by Leach et al (2008). Diffuse Galactic emission is described by a four component model of the interstellar medium comprising free-free, synchrotron, thermal dust and spinning dust and is based on Miville-Deschênes et al (2008, see Miville-Deschênes 2009, for a review).…”

Planck2013 results. III. LFI systematic uncertainties

“…To do this we computed a weighted sum of the three maps for each effect using weights obtained derived with a pixel-based ILC (internal linear combination) component separation method (Leach et al 2008;Planck Collaboration XII 2014). The ILC method implements direct variance minimisation exploiting the fact that the CMB component (in thermodynamic temperature units) is constant across frequencies, while foregrounds are characterised by non-thermal spectra.…”