WMAP data analysis

Mining and dissecting the WMAP data

The WMAP space mission, launched by NASA in june 2001, has made available to the scientific community high signal-to-noise, high resolution observations of the sky in 5 frequency bands ranging from 23 to 94 GHz.

I have worked on WMAP data to separate CMB from galactic and extragalactic foregrounds, to measure the temperature and polarisation power spectra of the CMB emission (see below), and to measure scaling laws that connect the SZ flux Y to the X-ray luminosity and mass of clusters (see my SZ cluster page).

Component separation

Component separation consists in identifying, in the observed frequency maps, what part of the radiation is due to which astrophysical process of emission. This is illustrated on a small patch of sky in the following figure, where we show how the WMAP K-band map at 23 GHz (top left) can be decomposed into a CMB part (top right), and a noisy foreground map (bottom left). The latter can be further processed to reduce contamination by instrumental noise, and obtain a clean foreground map (bottom right), here by Wiener-filtering the noisy foreground map with a latitude-dependent filter.


The extraction of the CMB map by needlet ILC is described below, and in this paper. The extraction of the foreground map has been done with Tuhin Ghosh while he spent a month at APC as a visiting student, and is described here.

A full sky, low foreground, high resolution CMB map from WMAP

We have developed a component separation method, the needlet ILC, to extract a CMB map from multifrequency observations such as those provided by the WMAP collaboration. We have applied this method to WMAP 5-year observations to obtain a very clean, high resolution map of CMB emission (paper available here). As illustrated in the following figure, this map compares favourably with previous work published by our colleagues from other institutions, including the ILC map produced by the WMAP collaboration. On the left, we see that the first ILC map obtained by the WMAP team is still somewhat contaminated by the galaxy at low galactic latitude (top) and noisy at high galactic latitude (bottom). An implementation of the ILC in harmonic space by Tegmark and collaborators (right) is less noisy at the galactic pole, but also contaminated in the galactic plane, although this contamination is mostly on lower scales (absent from the WMAP ILC, which has a 1 degree resolution). The needlet ILC map (middle column) is cleaner in the galactic plane, although not yet to the level that the CMB is recovered perfectly. This is not surprising, as the WMAP only has 5 frequency channels, and the galactic emission is very strong and complex in the galactic plane.


WMAP temperature and polarisation power spectra

Together with a postdoctoral fellow, Soumen Basak, we have applied the needlet ILC method to measure the temperature and polarisation power spectra of the CMB.

The analysis is done by first applying a needlet ILC to find the best set of linear combinations of needlet coefficients that reject foreground contaminations in maps of CMB intensity and of E and B-mode polarisation. These filters are then applied to each individual year of observation from the WMAP 7-year data set. Spectra are estimated as noise-weighted averages of (local, in pixel space) band-averaged covariances.

Results are published in the following papers:

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