First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Foreground Emission

TL;DR

This paper presents a comprehensive framework for separating the CMB from Galactic and extragalactic foregrounds in the first-year WMAP data. It introduces masks to exclude foreground-dominated regions, constructs CMB maps via internal linear combination and maximum entropy methods, and uses external templates to quantify and remove foreground components. The analysis finds a frequency- and region-dependent synchrotron index, a steep dust index around 2.2, and only a minor spinning-dust contribution, with SZ effects and unresolved point-source contamination being small in the relevant bands. Collectively, these results yield a CMB map with minimal foreground contamination and demonstrate that foreground residuals have a limited impact on the inferred CMB power spectrum, while providing detailed astrophysical insight into Galactic microwave emission and a catalog of 208 point sources.

Abstract

Full sky maps are made in five microwave frequency bands to separate the temperature anisotropy of the CMB from foreground emission. We define masks that excise regions of high foreground emission. The effectiveness of template fits to remove foreground emission from the WMAP data is examined. These efforts result in a CMB map with minimal contamination and a demonstration that the WMAP CMB power spectrum is insensitive to residual foreground emission. We construct a model of the Galactic emission components. We find that the Milky Way resembles other normal spiral galaxies between 408 MHz and 23 GHz, with a synchrotron spectral index that is flattest (beta ~ -2.5) near star-forming regions, especially in the plane, and steepest (beta ~ -3) in the halo. The significant synchrotron index steepening out of the plane suggests a diffusion process in which the halo electrons are trapped in the Galactic potential long enough to suffer synchrotron and inverse Compton energy losses and hence a spectral steepening. The synchrotron index is steeper in the WMAP bands than in lower frequency radio surveys, with a spectral break near 20 GHz to beta < -3. The modeled thermal dust spectral index is also steep in the WMAP bands, with beta ~ 2.2. Microwave and H alpha measurements of the ionized gas agree. Spinning dust emission is limited to < ~5% of the Ka-band foreground emission. A catalog of 208 point sources is presented. Derived source counts suggest a contribution to the anisotropy power from unresolved sources of (15.0 +- 1.4) 10^{-3} microK^2 sr at Q-band and negligible levels at V-band and W-band.

Figures (13)

• Figure 1: The solid curve is a histogram of pixel values in the K-band map. The dotted line is the symmetric reversal of the negative sky temperature portion of the solid curve about the peak value. The dashed curve is their difference or remainder, which is attributed to excess foreground emission. The peak of the remainder curve defines the K-band sky temperature cut-off for the mask that we call "Kp0". A series of masks with varying degrees of severity are likewise defined with "m" for "minus" and "p" for "plus", as shown. Temperature steps are defined by the rms half-width of the sky histogram for values less than the mode.
• Figure 2: A full sky map of selected WMAP standard masks. The masking is based on the K-band signal levels, as discussed in §\ref{['mask']}. The series of masks allows for a choice of cut severity. (Each successive mask includes the previous regions; e.g., the p4 mask includes all of the pixels in the p12 mask.)
• Figure 3: (a) A H$\alpha$ map of the sky, corrected for extinction. The correction is only an approximation and is especially unreliable for regions where $\tau>1$. These high opacity regions are roughly demarcated by the contour lines, which have been smoothed for clarity. (b) A full sky map of free-free emission based on the five bands of WMAP measurements. The map is the result of the MEM modeling process, as described in the text. (c) Pixel values in the MEM-derived free-free map are compared against their values in the H$\alpha$ map, with each point representing an average in H$\alpha$ intensity bins of $\Delta I = 8$ R. The lack of scatter in the fainter bins reflects fidelity to the prior assumption of 11.4 $\mu$K/R at K-band. The data plotted only includes pixels where the H$\alpha$ optical depth is estimated to be $<0.5$ and the K-band antenna temperature is $>0.2$ mK. The data are consistent with the $\mu$K/R values given in Table \ref{['tbl-1']} within $\sim\pm 12$% uncertainty, depending on the fitting method used.
• Figure 4: (a) The Haslam 408 MHz sky map is largely dominated by synchrotron emission. (b) The 23 GHz K-band WMAP map, also dominated by synchrotron emission, is more concentrated towards the plane than the 408 MHz Haslam map because flatter spectral index regions increasingly dominate at the higher observing frequencies. The steep spectral index North Galactic Spur, for example, is much less apparent at WMAP frequencies. The variable synchrotron spectral index across the sky renders the Haslam 408 MHz map an inaccurate tracer of synchrotron emission at microwave frequencies. (c) The spectral index map of $\beta(408\; {\rm MHz}, 23\; {\rm GHz}$) shows the flatter spectral index ($\beta \sim -2.5$) regions of active star-formation in the plane, where the cosmic ray electrons are generated. The steeper spectral index regions ($\beta \sim -3$) off the plane suggest the energy losses suffered by the cosmic ray electrons during the period of time required for their diffusion away from the star-formation regions of their origin. This spectral index map is dominated by synchrotron emission, but still contains free-free emission. It has been generated after setting zero-points based on cosecant fits to both maps, which provides an absolute zero-point for the WMAP map.
• Figure 5: (a) The FDS dust map at 94 GHz based on data from IRAS and COBE. This map is used as a prior in the MEM fit. (b) The full sky thermal dust map from the MEM procedure run on the five band WMAP data and shown for W-band. The morphology is found to be similar both to the expectation (prior), in (a) and to the synchrotron result map, in (c). (c) This full sky map of synchrotron emission referred to K-band is based on the five bands of WMAP measurements. The map is the result of the MEM modeling process, as described in the text. Note the rough similarity of the microwave synchrotron emission to the thermal dust emission. This is presumed to be a result of their common origin in regions of star-formation.