Wilkinson Microwave Anisotropy Probe
Internal Linear Combination (ILC) Map
HEALPix, nested, res 9 (Nside=512)
The Internal Linear Combination (ILC) map is formed from a weighted linear combination of 5 Smoothed I Maps in which the weights are chosen to maintain the CMB anisotropy signal while minimizing the Galactic foreground contribution. The weights are determined by minimizing the variance of the measured temperatures with the additional constraint that sum of the weights is equal to 1. To account for the spatial variation of the spectral indices of the various foreground components across the sky and in particular in the Galactic plane, the sky is divided into 12 regions: 11 regions within the Galactic plane and 1 covering the outer Galactic plane and high Galactic latitudes. The weights are calculated for each of the 12 regions and a full-sky ILC map is generated using the obtained coefficients after smoothing the region boundardies with a 1.5 degree kernel. As a final step, a 'bias' correction' based on Monte Carlo simulations is applied to to the ILC map.
On angular scales greater than ~10 degrees, we believe that the nine-year ILC map provides a reliable estimate of the CMB signal, with negligible instrument noise, over the full sky. However, we caution that on smaller scales there is a significant structure in the bias correction map that is still uncertain.
In addition to the ILC map, we provide a FITS-format map which defines the 12 sky regions used to construct the ILC. Full documentation is contained in the FITS header.