Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Galactic Foreground Emission

TL;DR

This study refines Galactic foreground separation in the five-year WMAP data using an MCMC-based pixel-wise fitting framework that jointly models synchrotron, free-free, and dust emission, with optional spinning-dust or synchrotron-steepening components for the Galactic plane. It cross-validates with ILC, MEM, and template-cleaning approaches, and provides per-pixel parameter posteriors, error maps, and goodness-of-fit metrics. Outside the Galactic plane, a simple three-component model performs well (χ²ν ≈ 1.1–1.2), while the plane requires more complex treatments to account for anomalous low-frequency emission, with spinning-dust contributing up to ~14–20% of Ka-band flux in some fits. The results have minimal impact on CMB extraction outside the plane, and the authors anticipate Planck-era data to further constrain foregrounds and polarization, enabling more robust cosmological inferences.

Abstract

We present a new estimate of foreground emission in the WMAP data, using a Markov chain Monte Carlo (MCMC) method. The new technique delivers maps of each foreground component for a variety of foreground models, error estimates of the uncertainty of each foreground component, and provides an overall goodness-of-fit measurement. The resulting foreground maps are in broad agreement with those from previous techniques used both within the collaboration and by other authors. We find that for WMAP data, a simple model with power-law synchrotron, free-free, and thermal dust components fits 90% of the sky with a reduced chi-squared of 1.14. However, the model does not work well inside the Galactic plane. The addition of either synchrotron steepening or a modified spinning dust model improves the fit. This component may account for up to 14% of the total flux at Ka-band (33 GHz). We find no evidence for foreground contamination of the CMB temperature map in the 85% of the sky used for cosmological analysis.

Figures (21)

• Figure 1: Comparison maps of the five-year masks versus the three-year masks. The new masks cover slightly more of the Galactic plane and cover more regions with low synchrotron but high free-free emission. The diamond-shaped features arise because the new processing mask has been defined to correspond to low-resolution ($N_\mathrm{side}$=16) pixels, so that the same processing mask can be used at all resolutions. Top: comparison of KQ85 with the three-year Kp2 mask. Middle: comparison of KQ75 with the three-year Kp0 mask. Bottom: comparison of KQ95 with the three-year Kp12 mask.
• Figure 2: A slice in parameter space of the surface nulled by the ILC coefficients, assuming a three-component foreground model with power-law spectral behavior, $T(\nu) = T_s \nu^{\beta_s} + T_f \nu^{2.14}+ T_d\nu^{\beta_d}$. Each line is for a single ILC region, denoted by number. The parameter space is $T_f/T_s$, $T_d/T_s$, $\beta_s$, $\beta_d$. For this plot the x-axis is $\beta_s$ and the y-axis is $T_d/T_s$. The parameters $T_f/T_s$ and $\beta_d$ are fixed at 0.7 and 1.8, respectively. Each color is a different ILC region. Despite the variety amongst ILC coefficients, they often null similar regions of parameter space.
• Figure 3: Comparison of MEM foreground modeling results from the WMAP three-year and five-year analyses. The first three panels show latitude profiles of antenna temperature for the individual foreground model components. The last panel compares the observed foreground emission spectrum (diamonds) with spectra of the total MEM model and the individual model components (line segments between WMAP frequencies), averaged over $20^{\circ} < \vert b \vert < 30^{\circ}$. The differences between the three-year and five-year model results are mainly due to differences in zero levels between the three-year and five-year maps, and are consistent with the three-year year estimated error of $\sim 4 \mu$K . The mean model brightness exceeds the mean observed brightness at the higher frequencies because the observed brightness is negative for some pixels and the model is constrained to be positive for each pixel. This is less apparent in the five-year results because there are fewer negative pixels.
• Figure 4: Five-year temperature maps with foregrounds reduced via template cleaning. All maps have had the five-year ILC estimate for the CMB subtracted, and have been degraded to $N_\mathrm{side} = 32$. Frequency bands shown are Q, V, and W. Compare to Figure 10 of Hinshaw et al. (2007). Outside the Galactic mask, the template cleaning reduces foregrounds to $\sim 15 \mu$K or less.
• Figure 5: The exact cold neutral medium (CNM) spinning dust spectrum as calculated by Draine & Lazarian (1998), an analytic fit to the model, and the ad hoc "shifted" model which better fits radio observations in the Galactic plane. The shifted model may represent a mix of warm neutral medium and warm ionized medium models, or another emission process entirely. The vertical axis is in units of antenna temperature, but the overall scale is arbitrary. Agreement between the exact model and the analytic approximation is better than 5% over the frequency range from 15 to 35 GHz. This is smaller than the fractional error from fits that include a spinning dust component.