Abstract
The polarized Galactic synchrotron and thermal dust emission constitutes a major tool in the study of the Galactic magnetic field (GMF) and in constraining its strength and geometry for the regular and turbulent components. In this paper, we review the modeling of these two components of the polarized Galactic emission and present our strategy for optimally exploiting the currently existing data sets. We investigate a Markov chain Monte Carlo (MCMC) method to constrain the model parameter space through maximum-likelihood analysis, focusing mainly on dust polarized emission. Relying on simulations, we demonstrate that our methodology can be used to constrain the regular GMF geometry. Fitting for the reduced Stokes parameters, this reconstruction is only marginally dependent of the accuracy of the reconstruction of the Galactic dust grain density distribution. However, the reconstruction degrades, apart from the pitch angle, when including a turbulent component on the order of the regular one as suggested by current observational constraints. Finally, we applied this methodology to a set of Planck polarization maps at 353 GHz to obtain the first MCMC based constrains on the large-scale regular-component of the GMF from the polarized diffuse Galactic thermal dust emission. By testing various models of the dust density distribution and of the GMF geometry, we prove that it is possible to infer the large-scale geometrical properties of the GMF. We obtain coherent three-dimensional views of the GMF, from which we infer a mean pitch angle of 27 degrees with 14% scatter, which is in agreement with results obtained in the literature from synchrotron emission.
Highlights
From a cosmological perspective, the characterization of the polarized diffuse Galactic emission from synchrotron and thermal dust is of prime importance
Simulated maps For the simulated data used in the following, we consider two dust density distribution models: the regular Logarithmic Spiral Arm (LSA) Galactic magnetic field (GMF) model and the turbulent GMF model, both presented in Appendix A
Relying on simulations, we investigate in a toy model if we can safely use the constraints obtained on the geometry of the GMF from the Galactic thermal dust emission analysis to help the fit of the Galactic synchrotron emission maps
Summary
The characterization of the polarized diffuse Galactic emission from synchrotron and thermal dust is of prime importance. A careful separation of these Galactic components is needed to safely compute the reduced synchrotron Stokes parameters qs = Qs/Is and us = Us/Is. In contrast, assuming that a good model of the geometrical structure of the GMF can be obtained from thermal dust emission, this model could be used to constrain the relativistic electron density distribution and of the GMF strength through a fit of the synchrotron data. Simulated maps For the simulated data used in the following, we consider two dust density distribution models: the regular Logarithmic Spiral Arm (LSA) GMF model and the (random) turbulent GMF model, both presented in Appendix A Using these parametric models and realistic Planck noise maps, we produced different sets of high-resolution maps that simulate the Planck 353-GHz linear polarization Stokes parameter maps.
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