摘要：Context.The analysis of the full-skyPlanckpolarization data at 850 μm revealed unexpected properties of theE- andB-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 bothEand (most of all)Bmodes 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 primordialBmodes of the CMB, the observed positive cross-correlation betweenTandBfor 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 betweenTandB, 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 theEandBmodes 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 theT–Esignal 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 observedT–Bcorrelation, we propose a new line of interpretation of thePlanckobservations 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 affectEandBmodes for isotropic magnetic-field configurations; II) helical magnetic fields threading interstellar filaments cannot reproduce thePlanckresults; and III) a weak helical left-handed magnetic field structure in the solar neighborhood may explain theT–Bcorrelation seen in thePlanckdata. Such a magnetic-field configuration would also account for the observed large-scaleT–Ecorrelation.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.
关键词：enISM: magnetic fieldsdustextinctionlocal insterstellar matterISM: structurecosmic background radiation
