A high resolution foreground cleaned CMB map from WMAP

TL;DR

The paper develops a blind, scale- and region-aware foreground cleaning method for WMAP data, building a linear estimator that preserves the CMB while minimizing combined foreground and detector-noise power. By implementing a Wiener-like weighting with weights ${\bf w}_{\ell} = {\bf C}_{\ell}^{-1}{\bf e}/({\bf e}^t{\bf C}_{\ell}^{-1}{\bf e})$ and partitioning the sky into nine regions, the authors produce a high-resolution all-sky CMB map at $13^{\prime}$ with reduced non-CMB contamination outside the Galactic plane, alongside a Wiener-filtered CMB map and CMB-free templates. The all-sky analysis shows general agreement with the WMAP ILC on large scales but improves small-scale power due to better noise handling, and reveals a Virgo-aligned quadrupole and octopole alignment with a dynamic quadrupole correction estimated from the Solar System motion. The work demonstrates a robust, model-blind approach to foreground removal with practical benefits for cross-correlation studies and cosmological interpretation, complementing the WMAP team's results.

Abstract

We perform an independent foreground analysis of the WMAP maps to produce a cleaned CMB map (available online) useful for cross-correlation with, e.g., galaxy and X-ray maps. We use a variant of the Tegmark & Efstathiou (1996) technique that is completely blind, making no assumptions about the CMB power spectrum, the foregrounds, WMAP detector noise or external templates. Compared with the foreground-cleaned internal linear combination map produced by the WMAP team, our map has the advantage of containing less non-CMB power (from foregrounds and detector noise) outside the Galactic plane. The difference is most important on the the angular scale of the first acoustic peak and below, since our cleaned map is at the highest (13') rather than lowest (49') WMAP resolution. We also produce a Wiener filtered CMB map, representing our best guess as to what the CMB sky actually looks like, as well as CMB-free maps at the five WMAP frequencies useful for foreground studies. We argue that our CMB map is clean enough that the lowest multipoles can be measured without any galaxy cut, and obtain a quadrupole value that is slightly less low than that from the cut-sky WMAP team analysis. This can be understood from a map of the CMB quadrupole, which shows much of its power falling within the Galaxy cut region, seemingly coincidentally. Intriguingly, both the quadrupole and the octopole are seen to have power suppressed along a particular spatial axis, which lines up between the two, roughly towards (l,b) \~ (-110,60) in Virgo.

Figures (15)

• Figure 3: The weights ${\bf w}_\ell$ used to create the internal linear combination map of the WMAP team are independent of angular scale. The figure shows the weights ${\bf w}_\ell = (0.109, -0.684, -0.096, 1.921, -0.250)$ used outside of the galactic plane.
• Figure 4: If there were no foregrounds and equal noise in the five input maps, then equal weighting at low $\ell$ would give way to favoring the highest resolution bands at high $\ell$. This example uses the forecast WMAP beam and noise specifications from foregpars rather than the actual ones.
• Figure 5: The optimal WMAP weights forecast by foregpars for the middle-of-the-road foreground model from foregpars.
• Figure 6: The actual weights we use for the 3rd cleanest of the 9 sky regions shown in Figure \ref{['masksFig']}. The dirtier the sky region, the more aggressive the weighting becomes, using large negative and positive values to subtract foregrounds.
• Figure 7: Sample band power window functions are shown for the cleanest of the sky regions from Figure \ref{['masksFig']}, all normalized so that the maximum value is unity. The approximate lack of leakage from odd numbers of multipoles away results from the approximate parity symmetry of this region.