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

TL;DR

This seven-year WMAP foreground study rigorously characterizes Galactic emission and validates foreground removal for CMB analysis by employing ILC, MEM, template cleaning, and pixel-based MCMC fitting across updated masks (KQ75y7 and KQ85y7). It finds no significant contamination outside the masks, supports a three-component model (synchrotron, free-free, dust) with possible spinning-dust contributions, and detects no compelling polarization haze. The work provides updated CMB maps, expanded point-source catalogs, and a thorough assessment of systematics through pipeline simulations and beam tests, strengthening the reliability of CMB results and informing low-frequency foreground modeling for future experiments.

Abstract

[Abridged] We present updated estimates of Galactic foreground emission using seven years of WMAP data. Using the power spectrum of differences between multi-frequency template-cleaned maps, we find no evidence for foreground contamination outside of the updated (KQ85y7) foreground mask. We place a 15 microKelvin upper bound on rms foreground contamination in the cleaned maps used for cosmological analysis. We find no indication in the polarization data of an extra "haze" of hard synchrotron emission from energetic electrons near the Galactic center. We provide an updated map of the cosmic microwave background (CMB) using the internal linear combination (ILC) method, updated foreground masks, and updates to point source catalogs with 62 newly detected sources. Also new are tests of the Markov chain Monte Carlo (MCMC) foreground fitting procedure against systematics in the time-stream data, and tests against the observed beam asymmetry. Within a few degrees of the Galactic plane, WMAP total intensity data show a rapidly steepening spectrum from 20-40 GHz, which may be due to emission from spinning dust grains, steepening synchrotron, or other effects. Comparisons are made to a 1-degree 408 MHz map (Haslam et al.) and the 11-degree ARCADE 2 data (Singal et al.). We find that spinning dust or steepening synchrotron models fit the combination of WMAP and 408 MHz data equally well. ARCADE data appear inconsistent with the steepening synchrotron model, and consistent with the spinning dust model, though some discrepancies remain regarding the relative strength of spinning dust emission. More high-resolution data in the 10-40 GHz range would shed much light on these issues.

Figures (13)

• Figure 1: Comparison of seven-year masks to five-year masks. At the top KQ75 and KQ75y7 are compared, and at the bottom KQ85 and KQ85y7. Green regions are masked in both the seven-year and five-year masks, yellow regions are newly masked in the seven-year masks, and red regions are masked in the five-year masks but no longer in the seven-year masks.
• Figure 2: Difference map between the seven-year ILC map and the five-year ILC map. Small-scale differences are consistent with pixel noise; large-scale differences are consistent with a change in dipole of $6.7$$\mu\textrm{K}$.
• Figure 3: Galactic signal component maps as determined by the Maximum Entropy Method (MEM) analysis. On the left are the input prior maps, and on the right are the output MEM maps. From top to bottom are the synchrotron, free-free, and dust components. While the output maps show many features of the prior at higher latitudes, there are clear differences in regions of strong emission.
• Figure 4: Difference maps between the seven-year MEM foreground maps and the five-year MEM foreground maps. Apart from a small dipole shift and noise fluctuations, the only visible feature is a small shift of $0.17\%$ of $K$-band flux from free-free to synchrotron.
• Figure 5: Galactic emission from two regions in the Galactic plane. ARCADE (triangles), WMAP (stars), and 408 MHz data (square) are all shown, smoothed to a common resolution. Upper panels show antenna temperature (absent a monopole component). The black line is a power-law connecting 408 MHz to 22 GHz ($\beta=-2.48$ for the left panel, $\beta=-2.41$ for the right panel), which is divided out in the bottom panels to better show deviations from power-law behavior. Red lines show the result of a fit to the data using three power law components for foregrounds (representing synchrotron, free-free, and dust). Blue lines show the fit resulting when an extra component representing spinning dust is added. Solid lines show the total flux, with individual components shown by dashed lines (synchrotron), dotted lines (free-free), and dot-dashed lines (dust plus spinning dust). Errors in the data are dominated by systematics and highly correlated between data points, but are estimated to be $5-15\%$, depending on experiment.