Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Foreground Polarization

TL;DR

This paper develops a pixel-based, two-component model for polarized Galactic foregrounds in WMAP three-year data, separating synchrotron and thermal dust contributions with spatially varying synchrotron spectra and dust polarization fractions. Using cross-variance and band-averaged maps across 23–94 GHz, the authors derive a mean synchrotron spectral index of $\beta_s \approx -3.2$ and a dust spectral index of $\beta_d = 2$, with dust polarization fractions of a few percent and synchrotron polarization rising to over 20% at high latitudes. The model shows that synchrotron dominates at low frequencies (below $\sim$60 GHz) and the two components account for at least 97% of polarized emission, while spinning-dust polarization remains undetectable. The approach improves foreground subtraction for CMB polarization studies and yields robust cosmological inferences with minimal foreground leakage into the polarization signal.

Abstract

We present a full-sky model of polarized Galactic microwave emission based on three years of observations by the Wilkinson Microwave Anisotropy Probe (WMAP) at frequencies from 23 to 94 GHz. The model compares maps of the Stokes Q and U components from each of the 5 WMAP frequency bands in order to separate synchrotron from dust emission, taking into account the spatial and frequency dependence of the synchrotron and dust components. This simple two-component model of the interstellar medium accounts for at least 97% of the polarized emission in the WMAP maps of the microwave sky. Synchrotron emission dominates the polarized foregrounds at frequencies below 50 GHz, and is comparable to the dust contribution at 65 GHz. The spectral index of the synchrotron component, derived solely from polarization data, is -3.2 averaged over the full sky, with a modestly flatter index on the Galactic plane. The synchrotron emission has mean polarization fraction 2--4% in the Galactic plane and rising to over 20% at high latitude, with prominent features such as the North Galactic Spur more polarized than the diffuse component. Thermal dust emission has polarization fraction 1% near the Galactic center, rising to 6% at the anti-center. Diffuse emission from high-latitude dust is also polarized with mean fractional polarization 0.036 +/- 0.011.

Figures (8)

• Figure 1: Estimated signal variance at each WMAP frequency band for 37 pixels. The solid line shows a two-component fit with power-law synchrotron and thermal dust emission assuming no spatial correlations between the two components. Allowing spatial correlations between the synchrotron and dust better reproduces the observed spectra (dotted lines). The thick dashed line shows the 68% confidence noise level. Note the change in scale between panels: the top panel shows high-latitude data outside the P06 sky mask, while the bottom panel shows data inside P06 dominated by emission from the Galactic plane.
• Figure 2: Spectral index of polarized synchrotron emission between 22 and 33 GHz (Mollweide projection in Galactic coordinates). The map has been smoothed with a position-dependent tophat varying from 7 radius near the plane to 18 radius at high latitude.
• Figure 3: Spectral index of polarized synchrotron emission binned by Galactic latitude. The spectral index steepens off the Galactic plane. Similar behavior is observed in external edge-on spiral galaxies.
• Figure 4: Polarized synchrotron model parameters. (Top) Antenna temperature $P = (Q^2 + U^2)^{0.5}$ of polarized synchrotron emission at K band. Vectors indicate the polarization direction. (Bottom) Synchrotron fractional polarization, derived by dividing the polarized model by the unpolarized maximum-entropy synchrotron model at K band. The polarization fraction is 3--5% on the Galactic plane, increasing above 20% within the North Galactic Spur and its southern extension.
• Figure 5: Histogram of synchrotron fractional polarization $f_s$. (Top) High latitude region outside the P06 mask. (Middle) Mid-latitude region inside P06 but with $|b| > 5\arcdeg$. (Bottom) Galactic plane, $|b| < 5\arcdeg$.