cleaned using templates
As demonstrated in the previous section, the W MAP data cleaned by foreground templates still show evidence of contamination due to either the inability of the templates to describe real spectral variations in the foreground emission on the sky, or other emission mechanisms not identified with the standard Galactic components/templates. Inspired by the analysis of Patanchon et al. (2005), we have performed an ‘iterative’ blind analysis on the W MAP data, after pre-cleaning with the foreground templates, in order to study the residuals in more detail.
Various combinations of the cleaned W MAP channels are used as input to the algorithm. This then returns a number of components equal to the number of the input sky maps. One of these is identified as a CMB map, cleaned as far as possible from any residual emission (see Section 3.8.2). Without exception, at most one of the other returned components can be clearly identified with Galactic residuals, the remainder generally being some combination of anisotropic noise (reflecting the underlying observation pattern of the W MAP satellite) and weak residual dipole emission. Figure 3.8 shows maps of the candidate foreground residuals for a number of W MAP and template input permutations, as described in the figure caption and discussed in Section 3.8.1
3.8.1 Foreground residuals
The upper row of Figure 3.8 shows the output maps from that we identify with foreground residuals when using the Haslam template for synchrotron emission.
The leftmost figure presents results from analysing the five W MAP frequency bands after cleaning using the coefficients previously derived in this chapter (see Table 3.1). There are some bright features along the Galactic plane near to the Galactic Centre and the presence of a structure related to the
Figure 3.7: Binned power spectra of the Q-, V- and W-maps cleaned from the foreground emission. Their contamination is described by the coupling coefficients determined bywith both the p- and g-functions (asterisk and diamond respectively) and by the W MAP analysis (triangle). The left figure corresponds to the case when the Haslam map is used as the synchrotron template, the right figure to the corresponding analysis using K-Ka for the low-frequency emission.
North Polar Spur. These are a clear indication of the deficiencies of the Haslam map as a synchrotron template, particularly for cleaning the K- and Ka-band data, with which the residuals are predominantly associated. Some of the positive and negative features presumably reflect either under- or over- subtraction of the true foreground signal by the templates as a consequence of spectral variations.
The middle figure shows the equivalent analysis using just the Q-, V- and W-band pre-cleaned sky maps. The residuals are significantly reduced, and no evidence of the North Polar Spur is now observed. The dominant feature, as before, is the structure close to the Galactic centre. This was originally identified and referred to as the ‘free-free haze’ by Finkbeiner (2004a) (although its origin as free-free emission is unlikely and it is now referred to as the ‘W MAP haze’), and was already visible in the foreground residuals plot of Bennett et al. (2003). Moreover, the map is strikingly (though perhaps unsurprisingly) similar to the map obtained by Patanchon et al. (2005), using Spectral Matching Independent Component Analysis (SMICA). They have associated these residual structures with the Ophiuchus complex, the Gum nebula, the Orion-Eridanus bubble and the Taurus region. The right-
Figure 3.8: Maps of the residual components obtained by when applied to the W MAP data after pre- cleaning using template fits with coupling coefficients α derived either using a analysis with the p- function and the K p2 mask or by the W MAP team in Bennett et al. (2003) (WMAP1) and Hinshaw et al. (2007) (WMAP3). Several cases have been considered: i) input maps K-, Ka-, Q-, V- and W-bands plus Haslam synchrotron template,coefficients; ii) input maps Q-, V- and W-bands plus Haslam synchrotron template, coefficients; iii) input maps Q-, V- and W-bands plus Haslam synchrotron template, W MAP constrained fit coefficients; iv) input maps Q-, V- and W-bands plus K-Ka synchrotron template,coefficients; v) input maps Q-, V- and W-bands plus K-Ka synchrotron template, W MAP constrained fit coefficients.
hand plot is the same analysis when the three input maps are cleaned using the W MAP foreground model of Bennett et al. (2003). The residual features are in excellent agreement with those from the middle plot, although the amplitudes are slightly larger, suggesting that thetemplate coefficients provide a modestly better cleaning of the W MAP frequency data.
The residual maps from input data cleaned with the K-Ka synchrotron model are shown in the second row of Figure 3.8. The maps can be compared directly to the equivalent Haslam cases in the row above. It is particularly striking that the residuals are substantially suppressed when using the K-Ka template, especially close to the Galactic Centre. This reinforces the latter’s superiority for foreground separation purposes, although the fact that it is a mixture of physical emission components makes the interpretation of the results more difficult. As before, the residuals are larger when using the W MAP template coefficients, notably so near the Gum nebula – a source of strong free-free emission.
3.8.2 CMB component
In order to quantify the improvement connected to the iterative processing step, we have studied the CMB components returned bywhen working on the data pre-cleaned with a template analysis stage. They are very similar, as shown in Figure 3.9. This is also confirmed by the plot of the power spectrum for the CMB map derived from the different cases of the foreground analysis. Figure 3.10
Figure 3.9: Map of the CMB component obtained bywith the p-function using W MAP Q-, V- and W- band data pre-cleaned using coupling coefficients estimated with the K p2 mask, and either the Haslam map as the synchrotron template (left) or the K-Ka map (right). The temperature is measured in mK.
Figure 3.10: Power spectra of the returned CMB components applyingto the set of Q-, V- and W-band data from the foreground emissions, using various combinations of foreground templates and fit coefficients.
compares the binned power spectrum of the CMB maps returned from the Q-, V- and W-band data cleaned using the coupling coefficients of the analysis or the W MAP values, with either the Haslam or K-Ka maps as synchrotron template. The discrepancy is less than 150µK2.
Given this, one might argue that the iterative step is unnecessary for W MAP analysis, but we fully expect that it will be essential for studying the Planck data in detail. Moreover, in this era of precision cosmology, it is the deviations from the theoretically motivated model spectra which are of interest and the search for evidence of primordial non-Gaussianity requires that false detections due to foreground residuals be eliminated. can play a role in the initial cleaning of the data, and then in defining regions on the sky that merit special treatment, be it simply masking them from future cosmological analysis due to the apparently complex nature of the foregrounds there, or the re-application of the component separation techniques on the regions to improve the final CMB sky estimate.