Planck intermediate results. XXX. The angular power spectrum of polarized dust emission at intermediate and high Galactic latitudes (1409.5738v2)

Abstract: The polarized thermal emission from Galactic dust is the main foreground present in measurements of the polarization of the cosmic microwave background (CMB) at frequencies above 100GHz. We exploit the Planck HFI polarization data from 100 to 353GHz to measure the dust angular power spectra $C_\ell^{EE,BB}$ over the range $40<\ell<600$ well away from the Galactic plane. These will bring new insights into interstellar dust physics and a precise determination of the level of contamination for CMB polarization experiments. We show that statistical properties of the emission can be characterized over large fractions of the sky using $C_\ell$. For the dust, they are well described by power laws in $\ell$ with exponents $\alpha^{EE,BB}=-2.42\pm0.02$. The amplitudes of the polarization $C_\ell$ vary with the average brightness in a way similar to the intensity ones. The dust polarization frequency dependence is consistent with modified blackbody emission with $\beta_d=1.59$ and $T_d=19.6$K. We find a systematic ratio between the amplitudes of the Galactic $B$- and $E$-modes of 0.5. We show that even in the faintest dust-emitting regions there are no "clean" windows where primordial CMB $B$-mode polarization could be measured without subtraction of dust emission. Finally, we investigate the level of dust polarization in the BICEP2 experiment field. Extrapolation of the Planck 353GHz data to 150GHz gives a dust power $\ell(\ell+1)C_\ell^{BB}/(2\pi)$ of $1.32\times10^{-2}\mu$K$_{CMB}^2$ over the $40<\ell<120$ range; the statistical uncertainty is $\pm0.29$ and there is an additional uncertainty (+0.28,-0.24) from the extrapolation, both in the same units. This is the same magnitude as reported by BICEP2 over this $\ell$ range, which highlights the need for assessment of the polarized dust signal even in the cleanest windows of the sky.

• The paper characterizes polarized dust EE and BB power spectra with a power-law exponent of –2.42 and a BB/EE ratio of 0.5.
• It employs classical and pure pseudo-Cℓ methods alongside simulations to ensure robust measurements over multipoles 40–600.
• The findings highlight that no sky region is free from dust contamination, stressing the importance of precise foreground subtraction in CMB B-mode searches.

Overview of the Analysis of Polarized Dust Emission by the Planck Collaboration

The paper "Planck Intermediate Results. XXX. The Angular Power Spectrum of Polarized Dust Emission at Intermediate and High Galactic Latitudes" presents a detailed analysis of the polarized thermal emission from Galactic dust, a significant foreground for observations of the Cosmic Microwave Background (CMB) polarization, particularly at frequencies above 100 GHz. This paper utilizes data from the Planck High-Frequency Instrument (HFI) to quantify the angular power spectra of dust polarization, denoted as EE and BB, across a wide range of multipoles (40 < ℓ < 600) at high Galactic latitudes.

Main Findings

1. Power Spectra Characterization:
   • The polarized dust EE and BB power spectra at 353 GHz are characterized as power laws in multipole moment ℓ, with exponents α_EE and α_BB accurately determined as −2.42±0.02. This consistency across various Galactic regions highlights a robust feature of dust polarization dynamics.
   • The paper acknowledges the systematic difference in amplitude between the EE and BB modes, with an observed ratio C_ℓ^{BB} / C_ℓ^{EE} = 0.5, challenging simple models of Galactic magnetic fields and dust alignment.
2. Frequency Dependence and Spectral Energy Distribution (SED):
   • The frequency dependence of the dust polarization spectra aligns with a modified blackbody model, characterized by β_d = 1.59 and T_d = 19.6 K. This relationship is paramount for understanding the impact of dust on CMB studies and facilitates accurate extrapolation to other frequencies.
3. Implications for CMB B-mode Searches:
   • The paper clarifies that even in the faintest dust-emitting regions, there are no "clean" regions on the sky free of dust contamination in the BB-mode polarization spectrum. This insight is crucial for future experiments aimed at detecting primordial gravitational waves through CMB B-modes, as it emphasizes the necessity of foreground subtraction.
4. Application to the BICEP2 Field:
   • The specific analysis of the field targeted by the BICEP2 experiment demonstrates a significant dust contribution to the observed BB power spectrum. The estimated dust power extrapolated to 150 GHz is comparable to the BICEP2 detections, underscoring the challenge of disentangling dust from cosmological signals.

Methodological Approaches

The methodology employs cross-correlation of detector sets to minimize noise bias and explores two computational approaches: the classical pseudo-C_ℓ method and the pure pseudo-C_ℓ estimator. Both methods are evaluated through simulations to ensure reliability and robustness against systematics. The analysis benefits from large-scale sky coverage achieved by Planck, allowing comprehensive assessment across varying sky fractions and frequencies.

Implications and Future Directions

The findings of this paper underline the necessity for enhanced characterization and mitigation of dust as a foreground for CMB polarization studies. The amplitude and variance of dust polarization highlight the complexity of Galactic foregrounds, suggesting that future ground-based CMB experiments should incorporate high-frequency channels for dust monitoring or rely on Planck's high-frequency data for improved foreground modeling.

This paper effectively addresses the critical issue of Galactic dust contamination, providing essential data and insights that demand rigorous component separation in CMB polarization research. The consistent spectral behavior observed across different sky patches not only informs observational strategies but also invites modeling refinements to account for the discrepancy between EE and BB amplitudes in dust polarization. As CMB experiments aim for greater sensitivity in detecting primordial signals, especially those indicative of inflationary processes, the analysis provided by the Planck collaboration represents a cornerstone in addressing the challenges posed by polarized dust.