Planckintermediate results

Abstract

Using data from the Planck satellite, we study the statistical properties of interstellar dust polarization at high Galactic latitudes around the south pole (b < −60• ). Our aim is to advance the understanding of the magnetized interstellar medium (ISM), and to provide a modelling framework of the polarized dust foreground for use in cosmic microwave background (CMB) component-separation procedures. We examine the Stokes I, Q, and U maps at 353 GHz, and particularly the statistical distribution of the polarization fraction (p) and angle (ψ), in order to characterize the ordered and turbulent components of the Galactic magnetic field (GMF) in the solar neighbourhood. The Q and U maps show patterns at large angular scales, which we relate to the mean orientation of the GMF towards Galactic coordinates (l 0 , b 0 ) = (70The histogram of the observed p values shows a wide dispersion up to 25%. The histogram of ψ has a standard deviation of 12• about the regular pattern expected from the ordered GMF. We build a phenomenological model that connects the distributions of p and ψ to a statistical description of the turbulent component of the GMF, assuming a uniform effective polarization fraction (p 0 ) of dust emission. To compute the Stokes parameters, we approximate the integration along the line of sight (LOS) as a sum over a set of N independent polarization layers, in each of which the turbulent component of the GMF is obtained from Gaussian realizations of a power-law power spectrum. We are able to reproduce the observed p and ψ distributions using a p 0 value of 26%, a ratio of 0.9 between the strengths of the turbulent and mean components of the GMF, and a small value of N. The mean value of p (inferred from the fit of the large-scale patterns in the Stokes maps) is 12 ± 1%. We relate the polarization layers to the density structure and to the correlation length of the GMF along the LOS. We emphasize the simplicity of our model (involving only a few parameters), which can be easily computed on the celestial sphere to produce simulated maps of dust polarization. Our work is an important step towards a model that can be used to assess the accuracy of component-separation methods in present and future CMB experiments designed to search the B mode CMB polarization from primordial gravity waves.Key words. magnetohydrodynamics (MHD) -polarization -methods: data analysis -dust, extinction -cosmic background radiation -ISM: magnetic fields IntroductionInterstellar magnetic fields are tied to the interstellar gas. Together with cosmic rays they form a dynamical system that Corresponding author: A. Bracco, e-mail: andrea.bracco@cea.fr is an important (but debated) facet of the physics of galaxies. Magnetic fields play a pivotal role because they control the density and distribution of cosmic rays, and they act on the dynamics if the gas. Much of the physics involved in this interplay is encoded in the structure of interstellar magnetic fields. Observations of synchrotron emission and its polarization, asArticle pu...

IMAGINE: a comprehensive view of the interstellar medium, Galactic magnetic fields and cosmic rays

“…The structure of the local magnetic field on scales of a few 100 pc, e.g., the Local Bubble [211] is currently not taken into account in existing GMF models. However, the analysis of the Planck dust polarisation maps stress the need to consider this local contribution in order to model polarisation observations away from the Galactic plane [204,212,213].…”

Universe opacity and CMB

“…The grain alignment by magnetic fields proved to be an efficient and rapid process which causes a linear polarization of starlight when passing through the dust and a polarized thermal emission of dust (Lazarian 2007). Hence, optical, IR and FIR polarimetry can reveal the presence and the detailed structure of magnetic fields in our Galaxy (Planck Collaboration et al 2015, 2016b as well as in large-scale formations in the Universe (Feretti et al 2012). The polarized light can be used not only for tracing magnetic fields but also for detecting dust.…”

“…Tracing magnetic fields in our Galaxy is particularly important for the analysis of the CMB because the polarized galactic dust forms a foreground which should be eliminated from the CMB polarization maps (Lazarian & Prunet 2002;Gold et al 2011;Ichiki 2014;Planck Collaboration et al 2015, 2016b. Some authors also point to a possible interaction of the CMB with intergalactic magnetic fields (Ohno et al 2003) and admit that these fields may modify the pattern of the CMB anisotropies and, eventually, induce additional anisotropies in the polarization (Giovannini 2004).…”

Exploring the Magnetic Field Geometry in NGC 891 with SOFIA/HAWC+

“…Most studies generate isotropic random magnetic fields using a Gaussian random field with a Kolmogorov-like power spectrum. The power spectrum represents the magnitude of turbulent magnetic energy depending on the physical scale of the turbulence, and the random magnetic fields are generally generated in the grid of a constant cell size (e.g., Jaffe et al 2010;Planck Collaboration et al 2016a, 2016b. Some models make an effort to match the models with observations by varying the amplitude of the random component depending on location in the Milky Way and adding an anisotropic random component (see details in Planck Collaboration et al 2016a).…”