Astrophysical component separation
CMB and foreground separation
The aim of Sevem is to produce clean CMB maps at one or several frequencies by using a procedure based on template fitting. The templates are internal, i.e., they are constructed from Planck data, avoiding the need for external data sets, which usually complicates the analyses and may introduce inconsistencies. The method has been successfully applied to Planck simulations Leach et al., 2008 and to WMAP polarisation data Fernandez-Cobos et al., 2012. In the cleaning process, no assumptions about the foregrounds or noise levels are needed, rendering the technique very robust.
The templates are constructed by subtracting two neighbouring Planck frequency channel maps, after first smoothing them to a common resolution to ensure that the CMB signal is properly removed. A linear combination of the templates is then subtracted from the Planck sky map at the frequency to be cleaned, in order to produce the clean CMB. The coefficients of the linear combination are obtained by minimising the variance of the clean map outside a given mask. Although we exclude very contaminated regions during the minimization, the subtraction is performed for all pixels and, therefore, the cleaned maps cover the full-sky (although we expect that foreground residuals are present in the excluded areas).
An additional level of flexibility can also be considered: the linear coefficients can be the same for all the sky, or several regions with different sets of coefficients can be considered. The regions are then combined in a smooth way, by weighting the pixels at the boundaries, to avoid discontinuities in the clean maps. In order to take into account the different spectral behaviour of the foregrounds at low and high galactic latitudes, we have chosen to use two regions: the region with the 3 per cent brightest Galactic emission, and the region with the remaining 97 per cent of the sky.
The final CMB map has then been constructed by combining the 143 and 217 GHz cleaned maps by weighting the maps in harmonic space taking into account the noise level, the resolution and a rough estimation of the foreground residuals of each map (obtained from realistic simulations). This final map has a resolution corresponding to a Gaussian beam of FWHM =5 arcminutes.
Cosmic Microwave background