Planck 2018 results. XI. Polarized dust foregrounds

TL;DR

Planck PR3 polarization maps enable a comprehensive characterization of polarized Galactic dust as a foreground to CMB polarization. The authors quantify dust EE/BB/TE/etc. power spectra across six high-latitude sky regions, confirm an E/B asymmetry and a persistent TE correlation down to large angular scales, and report a positive TB signal without evidence of instrumental leakage. By combining Planck with WMAP data, they model the microwave SED of polarized dust and synchrotron, finding a single-temperature dust SED with $\beta_{d}^{P} = 1.53 \pm 0.02$ and no conclusive decorrelation across 353 GHz to below 70 GHz, which constrains foreground modelling for primordial B-mode searches. The results imply that, for current sensitivity targets around $r \approx 0.01$, frequency decorrelation is unlikely to be a dominant obstacle, though regional variations and future higher-precision experiments require continued development of dust models and multi-frequency analyses.

Abstract

The study of polarized dust emission has become entwined with the analysis of the cosmic microwave background (CMB) polarization. We use new Planck maps to characterize Galactic dust emission as a foreground to the CMB polarization. We present Planck EE, BB, and TE power spectra of dust polarization at 353 GHz for six nested sky regions covering from 24 to 71 % of the sky. We present power-law fits to the angular power spectra, yielding evidence for statistically significant variations of the exponents over sky regions and a difference between the values for the EE and BB spectra. The TE correlation and E/B power asymmetry extend to low multipoles that were not included in earlier Planck polarization papers. We also report evidence for a positive TB dust signal. Combining data from Planck and WMAP, we determine the amplitudes and spectral energy distributions (SEDs) of polarized foregrounds, including the correlation between dust and synchrotron polarized emission, for the six sky regions as a function of multipole. This quantifies the challenge of the component separation procedure required for detecting the reionization and recombination peaks of primordial CMB B modes. The SED of polarized dust emission is fit well by a single-temperature modified blackbody emission law from 353 GHz to below 70 GHz. For a dust temperature of 19.6 K, the mean spectral index for dust polarization is $β_{\rm d}^{P} = 1.53\pm0.02 $. By fitting multi-frequency cross-spectra, we examine the correlation of the dust polarization maps across frequency. We find no evidence for decorrelation. If the Planck limit for the largest sky region applies to the smaller sky regions observed by sub-orbital experiments, then decorrelation might not be a problem for CMB experiments aiming at a primordial B-mode detection limit on the tensor-to-scalar ratio $r\simeq0.01$ at the recombination peak.

Figures (29)

• Figure 1: All-sky map showing the sky regions used to measure power spectra, indicated with colours varying from yellow to orange and dark-red. The white region represents the area where the CO line brightness is larger than $0.4$Kkms$^{-1}$, which is excluded from all the sky regions in our analysis. The blue dots represent the areas masked around point sources.
• Figure 2: CMB-corrected $EE$ (red diamonds), $BB$ (blue squares), and $TE$ (black circles) power spectra at 353GHz, for each of the six sky regions that we analyse. The dashed lines represent power-law fits to the data points from $\ell = 40$ to $600$. The exponents of these fits, $\alpha_{\rm TE}$, $\alpha_{\rm EE}$, and $\alpha_{\rm BB}$, appear on each panel.
• Figure 3: Power spectra, as in Fig. \ref{['fig:spectra']}, but for the northern and southern parts of the LR42, LR52, LR62, and LR71 regions.
• Figure 4: Scaling of the $BB$ power at $\ell = 80$ versus the mean dust total intensity at 353GHz. The dashed black line is a power-law fit to values for the six sky regions in our analysis (blue dots) with an exponent of two. Also shown are the values for the N--S splits of the regions in Fig. \ref{['fig:spectra2']} (blue triangles). These results are complemented by the measurement (red diamond) over the southern Galactic cap ($f_{\rm sky} = 8.5$%) by Ghosh16 and that for the BICEP field (black square) after PhysRevLett.116.031302.
• Figure 5: $TE$ correlation ratio $r^{TE}_\ell$ versus multipole. The data points are plotted using distinct symbols and colours (see legend at the top) for each of the six sky regions. The error bars are derived from the E2E simulations.