Wilkinson Microwave Anisotropy Probe

Product Name
Sky Maps
Coord. System
Projection Type
HEALPix, nested
0.23°- 0.93° (frequency dependent)


WMAP maps of the sky are supplied in FITS files, with the maps contained within binary table extensions. The maps are stored in a nested HEALPix projection and are supplied at three resolutions: WMAP's standard Res 9 (NSide=512), Res 4 (NSide=16), and Res 10 (NSide=1024) for high-l TT analysis. Stokes I (temperature), Stokes Q and U (Polarization), and bandpass mismatch (a so-called S map) maps may be supplied in a file. Also included in the maps is an Nobs map describing the number of observations used to construct each pixel. A detailed description of how these files are formatted is available here.

Iterative algorithms are used to create skymaps from the calibrated differential time-ordered WMAP data for each of the ten differencing assemblies. Each pixel in a map represents a sky temperature for the bandpass appropriate to the differencing assembly. The CMB dipole has been removed from the Stokes I maps.

Maps for nine years of WMAP data have been supplied to LAMBDA. Full nine year maps were produced by performing a weighted, pixel-by-pixel, mean of the nine single year maps, with pixel weights given by the Nobs vector. For the Stokes I maps, the mean temperature outside of a preliminary version of the temperature analysis/KQ85 mask was subtracted from each single year map before the nine year weighted mean was calculated. The zero point of each nine year Stokes I map has been set using a Galactic csc |b| model. The zero level of each single year map has not been set, and is not a meaningful quantity. The Nobs measurements were added together to produce the final Nobs measurement. These maps have also been smoothed to 1° resolution; these smoothed maps have been supplied as a separate data product.

The maps of the individual differencing assemblies of a single frequency band (Q1 and Q2; V1 and V2; W1, W2, W3, and W4) have been combined to produce coadded maps for each frequency band.

Many of the files that contain polarization maps also contain an extension containing the polarization noise covariance matrices for the maps. See the detailed description mentioned above for formatting details. While available for the Res 4 maps, these matrices do not fully characterize the pixel-to-pixel noise in these maps. Therefore a more accurate and much larger inverse covariance (or N-1) matrix is supplied as a separate product.

Pixel noise in units of mK may be evaluated from Nobs with the expression σ = σ0 / sqrt(Nobs) where

σ0 For Each Differencing Assembly
K1 Ka1 Q1 Q2 V1 V2 W1 W2 W3 W4
Stokes I 1.429 1.466 2.245 2.131 3.314 2.949 5.899 6.562 6.941 6.773
Stokes Q,U, uncleaned 1.435 1.472 2.254 2.140 3.324 2.958 5.912 6.577 6.958 6.795
Stokes Q,U, foreground cleaned N/A 2.166 2.710 2.572 3.534 3.144 6.157 6.850 7.246 7.076

σ0 For Each Frequency Band
K Ka Q V W
Stokes I 1.429 1.466 2.188 3.131 6.544
Stokes Q,U, uncleaned 1.435 1.472 2.197 3.141 6.560
Stokes Q,U, foreground cleaned N/A 2.166 2.641 3.339 6.832

Foreground reduced maps have been produced by removing a foreground model from the 'unreduced' maps using the Foreground Template Model discussed in Bennett, et.al., 2013. The cleaning algorithm for Stokes I was updated for the nine-year data release. The Stokes I cleaning algorithm for previous releases is described in Hinshaw, et.al, 2007. The Stokes Q and U cleaning algorithm is described in Bennett, et.al., 2013 and Page, et.al, 2007; it hasn't changed significantly for the nine-year relase. Briefly, synchrotron, free-free, and dust emission templates were modeled and then subtracted from the single year 'unreduced' maps. The goal was to produce a set of maps with the foreground removed while retaining WMAP's noise characteristics.

K and Ka maps were used to produce the Stokes I foreground cleaned maps; therefore only the Q, V, and W frequency bands are provided for this product. K maps were used to produce the Stokes Q and U foreground cleaned maps, so only Ka, Q, V, and W frequency bands are provided.

As with the 'unreduced' maps, full nine year maps were produced by performing a weighted, pixel-by-pixel, mean of the nine single year maps; the Nobs measurement was used to weight this mean. The Nobs measurements were added together to produce the final, nine year, Nobs measurement.

The same foreground template model was used to produce foreground reduced high resolution (res 10) Q, V and W maps.

A new product was developed for the nine-year data release. A set of Stokes I maps were processed to reduce the asymmetry of the effective beam; the process deconvolves the maps using only the asymmetric component of the beam. The resulting map contains the true sky signal effectively convolved with a circularly symmetric beam. See Bennett, et.al., 2013 for details.

The low resolution (res 4, NSide=16) maps were created from the res 9 maps. The individual year maps were computed through a least-squares weighted boxaverage, treating the Stokes I maps separately from the Stokes Q, U, and S maps: QU, QS, ans US covariances were taken into account but IQ, IU and IS covarainces were omitted. The Nobs vectors were replaced with the appropriate N-1 diagonal elements scaled by σ02. The nine year low resolution maps were created through weighted co-addition of the individual year maps.

Additional Information

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A service of the HEASARC and of the Astrophysics Science Division at NASA/GSFC
Goddard Space Flight Center, National Aeronautics and Space Administration
HEASARC Director: Dr. Alan P. Smale
LAMBDA Director: Dr. Eric R. Switzer
NASA Official: Dr. Eric R. Switzer
Web Curator: Mr. Michael R. Greason