Foreground removal by an Internal Linear Combination method: limitations and implications

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  • 3 March 2004
Abstract
We study the Internal Linear Combination (ILC) method presented by the WMAP science team, with the goal of determining whether it may be used for cosmological purposes, as a template-free alternative to existing foreground correction methods. We conclude that the method does have the potential to do just that, but great care must be taken both in implementation, and in a detailed understanding of limitations due to residual foregrounds which can still affect cosmological results. As a first step we demonstrate how to compute the ILC weights both accurately and efficiently by means of Lagrange multipliers, and apply this method to the observed data to produce a new version of the ILC map. This map has 12% lower variance than the official ILC map, primarily due to less noise. Next we describe how to generate Monte Carlo simulations of the ILC map, and find that these agree well with the observed map on angular scales up to l~200 on a conservative sky cut. Finally we make two comments to the on-going debates concerning the large-scale properties of the WMAP data. First, we note that the Galactic south-eastern quadrant is associated with notably different ILC weights than the other three quadrants, possibly indicating a foreground related anisotropy. Second, we study the properties of the quadrupole and octopole (amplitude, alignment and planarity), and find that residual foregrounds do affect even the largest scales significantly. In particular, we use Monte Carlo simulations to assess the uncertainties connected to these measurements, and find that these are too large to allow for cosmological conclusions at this time.

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