Planckintermediate results

Abstract

The polarized thermal emission from diffuse Galactic dust is the main foreground present in measurements of the polarization of the cosmic microwave background (CMB) at frequencies above 100 GHz. In this paper we exploit the uniqueness of the Planck HFI polarization data from 100 to 353 GHz to measure the polarized dust angular power spectra C EE and C BB over the multipole range 40 < < 600 well away from the Galactic plane. These measurements will bring new insights into interstellar dust physics and allow a precise determination of the level of contamination for CMB polarization experiments. Despite the non-Gaussian and anisotropic nature of Galactic dust, we show that general statistical properties of the emission can be characterized accurately over large fractions of the sky using angular power spectra. The polarization power spectra of the dust are well described by power laws in multipole, C ∝ α , with exponents α EE,BB = −2.42 ± 0.02. The amplitudes of the polarization power spectra vary with the average brightness in a way similar to the intensity power spectra. The frequency dependence of the dust polarization spectra is consistent with modified blackbody emission with β d = 1.59 and T d = 19.6 K down to the lowest Planck HFI frequencies. We find a systematic difference between the amplitudes of the Galactic B-and E-modes, C BB /C EE = 0.5. We verify that these general properties are preserved towards high Galactic latitudes with low dust column densities. We show that even in the faintest dust-emitting regions there are no "clean" windows in the sky where primordial CMB B-mode polarization measurements could be made without subtraction of foreground emission. Finally, we investigate the level of dust polarization in the specific field recently targeted by the BICEP2 experiment. Extrapolation of the Planck 353 GHz data to 150 GHz gives a dust power D BB ≡ ( + 1)C BB /(2π) of 1.32 × 10 −2 µK 2 CMB over the multipole range of the primordial recombination bump (40 < < 120); the statistical uncertainty is ±0.29 × 10 −2 µK 2 CMB and there is an additional uncertainty (+0.28, −0.24) × 10 −2 µK 2 CMB from the extrapolation. This level is the same magnitude as reported by BICEP2 over this range, which highlights the need for assessment of the polarized dust signal even in the cleanest windows of the sky.

No-scale supergravity inflation: A bridge between string theory and particle physics?

“…There was great excitement when the BICEP2 experiment reported 7 the measurement of B-mode polarization in the CMB, which might have been generated by primordial tensor (quantum gravitational-wave) perturbations. However, these are now thought to be consistent with pollution by dust 8 , and a recent combined analysis of BICEP2/Keck array and Planck data yields only an upper limit on r < 0.1 9 , as seen in Fig. 1, where we also see that n s ∼ 0.97.…”

Line-of-sight extrapolation noise in dust polarization

“…. On the other hand, the expected CMB primordial Bmode power at = 80 for a tensor-to-scalar ratio of r = 1 is 6.71 × 10 −2 µK 2 CMB [6]. The primordial B-mode power scales linearly with r, so a CMB primordial B-mode spectrum at = 80 for r ≈ 0.0015 would have a power of 10 −4 µK 2 CMB , comparable to the noise contribution from the line-of-sight extrapolation noise.…”

“…At degree angular scales and frequencies above 100 GHz, the primary foreground contribution comes from linearly polarized emission from asymmetric dust grains that are aligned with the Galactic magnetic field [5][6][7][8].…”

“…In those analyses, the polarized dust SED was determined to be well-fitted by a single mean dust temperature and spectral index (e.g. see [6,9]). However, for the frequencies of interest in CMB experiments, the thermal SED for typical dust temperatures of ∼20 K is not fully in the Rayleigh-Jeans regime.…”

Time to move on?

“…There was great excitement when the BICEP2 experiment reported 10 the measurement of B-mode polarisation in the CMB, which might have been generated by primordial tensor (quantum gravitational-wave) perturbations. However, these are now thought to consistent with pollution by dust, 11 and a recent combined analysis of BICEP2/Keck array and Planck data yields only an upper limit on r < 0.1, 12 as seen in Fig. 1, where we also see that n s ∼ 0.97.…”