On Foreground Removal from the Wilkinson Microwave Anisotropy Probe Data by an Internal Linear Combination Method: Limitations and Implications

摘要

We study the Internal Linear Combination (ILC) method presented by theWilkinson Microwave Anisotropy Probe (WMAP) science team, with the goal ofdetermining whether it may be used for cosmological purposes, as atemplate-free alternative to existing foreground correction methods. Weconclude that the method does have the potential to do just that, but greatcare must be taken both in implementation, and in a detailed understanding oflimitations caused by residual foregrounds which can still affect cosmologicalresults. As a first step we demonstrate how to compute the ILC weights bothaccurately and efficiently by means of Lagrange multipliers, and apply thismethod to the observed data to produce a new version of the ILC map. Next wedescribe how to generate Monte Carlo simulations of the ILC map, and find thatthese agree well with the observed map on angular scales up to l~200. Finallywe make two comments to the on-going debates concerning the large-scaleproperties of the WMAP data. First, we note that the Galactic south-easternquadrant is associated with notably different ILC weights than the other threequadrants, possibly indicating a foreground related anisotropy. Second, westudy the properties of the quadrupole and octopole (amplitude, alignment andplanarity), and confirm the previously reported anomalies. Even moreinterestingly, we find that the l=5 mode is spherically symmetric at about 3sigma, and that the l=6 mode is planar at the 2 sigma level. However, we alsoassess the impact of residual foregrounds on these statistics, and find thatthe ILC map is not clean enough to allow for cosmological conclusions.Alternative methods must be developed to study these issues further.