Abstract
Context. The analysis of the full-sky Planck polarization data at 850 μm revealed unexpected properties of the E- and B-mode power spectra of dust emission in the interstellar medium (ISM). The positive cross-correlations over a wide range of angular scales between the total dust intensity, T, and both E and (most of all) B modes has raised new questions about the physical mechanisms that affect dust polarization, such as the Galactic magnetic field structure. This is key both to better understanding ISM dynamics and to accurately describing Galactic foregrounds to the polarization of the cosmic microwave background (CMB). In particular, in the quest to find primordial B modes of the CMB, the observed positive cross-correlation between T and B for interstellar dust requires further investigation towards parity-violating processes in the ISM. Aims. In this theoretical paper we investigate the possibility that the observed cross-correlations in the dust polarization power spectra, and specifically the one between T and B, can be related to a parity-odd quantity in the ISM such as the magnetic helicity. Methods. We produce synthetic dust polarization data, derived from 3D analytical toy models of density structures and helical magnetic fields, to compare with the E and B modes of observations. We present several models. The first is an ideal fully helical isotropic case, such as the Arnold-Beltrami-Childress field. Second, following the nowadays favored interpretation of the T–E signal in terms of the observed alignment between the magnetic field morphology and the filamentary density structure of the diffuse ISM, we design models for helical magnetic fields wrapped around cylindrical interstellar filaments. Lastly, focusing on the observed T–B correlation, we propose a new line of interpretation of the Planck observations advocating the presence of a large-scale helical component of the Galactic magnetic field in the solar neighborhood. Results. Our analysis shows that: I) the sign of magnetic helicity does not affect E and B modes for isotropic magnetic-field configurations; II) helical magnetic fields threading interstellar filaments cannot reproduce the Planck results; and III) a weak helical left-handed magnetic field structure in the solar neighborhood may explain the T–B correlation seen in the Planck data. Such a magnetic-field configuration would also account for the observed large-scale T–E correlation. Conclusions. This work suggests a new perspective for the interpretation of the dust polarization power spectra that supports the imprint of a large-scale structure of the Galactic magnetic field in the solar neighborhood.
Highlights
Recent analyses of the sub-millimeter emission observed with the Planck1 satellite (Planck Collaboration I 2016) showed that the linearly polarized light of Galactic interstellar dust is an unavoidable foreground for detecting the imprint of primordial gravitational waves on the polarization of the cosmic microwave background
Unless we consider an optical depth dependence of dust polarization, our toy models of helical magnetic fields around interstellar filaments are not able to probe the T –B correlation observed in the Planck data
We have presented toy models of helical magnetic fields in the ISM to gain insight into recent Planck observational results concerning Galactic dust polarization power spectra, or the positive T –E and T –B correlations, with particular focus on the latter
Summary
Recent analyses of the sub-millimeter emission observed with the Planck satellite (Planck Collaboration I 2016) showed that the linearly polarized light of Galactic interstellar dust is an unavoidable foreground for detecting the imprint of primordial gravitational waves on the polarization of the cosmic microwave background (CMB; i.e., BICEP2/Keck Collaboration & Planck Collaboration 2015; Planck Collaboration Int. XXX 2016, hereafter PIPXXX). In order to reach such a tremendous achievement, an accurate model of the Galactic polarized emission is required. Despite being discovered in the middle of the twentieth century with the first starlight polarization measurements (Hiltner 1949; Hall 1949), because of the complexity and the variety of physical processes at play, a benchmark model for the polarization of Galactic dust is still missing. The acknowledged mechanism responsible for dust polarization can be summarized as follows: due to their asymmetric-elongated shape, spinning velocities, size-distribution, composition, and optical properties, large interstellar grains, from micrometer (μm) to millimeter (mm) size, tend to align their axis of maximal inertia (the shortest axis) with the ambient magnetic field in the interstellar medium (ISM; Chandrasekhar & Fermi 1953) under the action of mechanical/radiative/magnetic torques
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