The Galactic magnetic field as revealed by Planck

ESA's Planck satellite has produced the first all-sky map of the polarized emission from dust at submm wavelengths. Compared with earlier ground-based and balloon-borne observations this survey is an immense step forward in sensitivity, coverage, and statistics. It provides new insight into the structure of the Galactic magnetic field and the properties of dust, as well as the first statistical characterization of one of the main foregrounds to CMB polarization.

Here we summarize the main results from the data analysis by the Planck Consortium on the structure of the Galactic magnetic field. We hope the release of the data to the science community at large will trigger many more studies.


The structure of the Galactic magnetic field from dust polarization maps of the southern Galactic cap

We study the statistical properties of interstellar dust polarization at high Galactic latitude, using the Stokes parameter Planck maps at 353 GHz. Our aim is to advance the understanding of the magnetized interstellar medium (ISM), and to provide a model of the polarized dust foreground for cosmic microwave background component-separation procedures. Focusing on the southern Galactic cap, we examine the statistical distributions of the polarization fraction (p) and angle (ψ) to characterize the ordered and turbulent components of the Galactic magnetic field (GMF) in the solar neighbourhood.

We relate patterns at large angular scales in polarization to the orientation of the mean (ordered) GMF towards Galactic coordinates (l0,b0)=(70°±5°,24°±5°). The histogram of p 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 use these histograms to build a phenomenological model of the turbulent component of the GMF, assuming a uniform effective polarization fraction (p0) of dust emission. To model 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 p0 value of (26±3)%; a ratio of 0.9±0.1 between the strengths of the turbulent and mean components of the GMF; and a small value of N. We relate the polarization layers to the density structure and to the correlation length of the GMF along the LOS.

Planck intermediate results. XLIV. The structure of the Galactic magnetic field from dust polarization maps of the southern Galactic cap

Orthographic projection centred on the south Galactic pole showing the magnetic field orientation inferred from the Planck 353 GHz polarization observations. The highlighted region corresponds to b<60°.

E- and B-modes of dust polarization from the magnetized filamentary structure of the interstellar medium

Radial profiles of the mean stacked Planck D353, Q'353, and U'353 images as functions of distance from the centre of the filament (blue line).

We present a statistical study of the filamentary structure of the 353GHz Planck Stokes maps at high Galactic latitude, relevant to the study of dust emission as a polarized foreground to the CMB. We filter the intensity and polarization maps to isolate filaments in the range of angular scales where the power asymmetry between E-modes and B-modes is observed.

Using the Smoothed Hessian Major Axis Filament Finder, we identify 259 filaments at high Galactic latitude, with lengths larger or equal to 2 degrees (corresponding to 3.5 pc in length for a typical distance of 100 pc). These filaments show a preferred orientation parallel to the magnetic field projected onto the plane of the sky, derived from their polarization angles. We present mean maps of the filaments in Stokes I, Q, U, E, and B, computed by stacking individual images rotated to align the orientations of the filaments. Combining the stacked images and the histogram of relative orientations, we estimate the mean polarization fraction of the filaments to be 11%.

Furthermore, we show that the correlation between the filaments and the magnetic field orientations may account for the E and B asymmetry and the CTE/CEE ratio, reported in the power spectra analysis of the Planck 353GHz polarization maps. Future models of the dust foreground for CMB polarization studies will need to take into account the observed correlation between the dust polarization and the structure of interstellar matter.

Planck intermediate results. XXXVIII. E- and B-modes of dust polarization from the magnetized filamentary structure of the interstellar medium


Probing the role of the magnetic field in the formation of structure in molecular clouds

Within ten nearby (less than 450 pc from the Sun) molecular clouds we evaluate statistically the relative orientation between the magnetic field projected on the plane of sky, inferred from the polarized thermal emission of Galactic dust observed by Planck at 353 GHz, and the gas column density structures, quantified by the gradient of the column density, NH.

The relative orientation is evaluated pixel by pixel and analyzed in bins of column density using the novel statistical tool called Histogram of Relative Orientations.

Within most clouds we find that the relative orientation changes progressively with increasing NH, from preferentially parallel or having no preferred orientation to preferentially perpendicular. In simulations of magnetohydrodynamic turbulence in molecular clouds this trend in relative orientation is a signature of Alfvenic or sub-Alfvenic turbulence, implying that the magnetic field is significant for the gas dynamics at the scales probed by Planck.

Planck intermediate results. XXXV. Probing the role of the magnetic field in the formation of structure in molecular clouds

Magnetic field and column density measured by Planck towards the Taurus molecular cloud.

The relative orientation between the magnetic field and structures traced by interstellar dust

Map and contours of the degree of alignment as a function of p and NH.

The formation of density structures in the interstellar medium involves turbulence, gas cooling, magnetic fields, and gravity. Polarization of thermal dust emission is well suited to studying the role of the magnetic field, because it images structure through an emission process that traces the mass of interstellar matter (Planck Collaboration XI 2014). The Planck I map shows elongated structures (filaments or ridges) that have counterparts in either the Stokes Q or U map, or in both, depending on the mean orientation.

The correlation between Stokes maps characterizes the relative orientation between the ridges and the magnetic field. In the diffuse interstellar medium, the ridges are preferentially aligned with the magnetic field measured on the structures. This statistical trend becomes more striking for decreasing column density and, as expected from the potential effects of projection, for increasing polarization fraction.

Planck intermediate results. XXXII. The relative orientation between the magnetic field and structures traced by interstellar dust


Signature of the magnetic field geometry of interstellar filaments in dust polarization maps

We present the polarization maps of three nearby star forming filament of moderate column density (NH~10^22cm^-2): Musca, B211, and L1506. We use the spatial information to separate Stokes I, Q, and U of the filaments from those of their backgrounds, an essential step to measure the intrinsic polarization fraction (p) and angle (psi) of each emission component. We find that the polarization angles in the three filaments (psi_fil) are coherent along their lengths and not the same as in their backgrounds (psi_bg). The differences between psi_fil and psi_bg are 12, 6, and 54 degrees for Musca, B211, and L1506, respectively. These differences for Musca and L1506 are larger than the dispersions of psi, both along the filaments and in their backgrounds.

The observed changes of psi are direct evidence for variations of the orientation of the plane of the sky (POS) projection of the B-field. As in previous studies, we find a decrease of several percent of p with NH. We show that the drop in p cannot be explained by random fluctuations of the orientation of B within the filaments because they are too small (sigma_psi less than 10 degrees). We recognize the degeneracy between dust alignment efficiency and the structure of B in causing variations in p, but we argue that the decrease of p from the backgrounds to the filaments results in part from depolarization associated with the 3D structure of B: both its orientation in the POS and with respect to the POS. We do not resolve the inner structure of the filaments, but at the smallest scales accessible with Planck (~0.2pc), the observed changes of psi and p hold information on the B-field structure within filaments.

Planck intermediate results. XXXIII. Signature of the magnetic field geometry of interstellar filaments in dust polarization maps (updated version coming up soon)

Planck 353 GHz Stokes parameter maps of the Musca filament (in MJy sr-1).

The magnetic field structure in the Rosette Nebula

The Rosette Nebula and its molecular cloud. Planck 353 GHz emission at 6' resolution. The black star indicates the position of the central star cluster NGC 2244 and the black circles correspond to the inner and outer radii of the HII region.

Planck Collaboration Int. XXXIV (2015) analyzes the magnetic field in a massive star forming region, the Rosette Nebula and parent molecular cloud, combining Faraday rotation measures from the ionized gas with dust polarized emission from the swept-up shell. This same methodology and modelling framework could be used to study the field structure in a sample of massive star forming regions.

Planck intermediate results. XXXIV. The magnetic field structure in the Rosette Nebula


Comparison of polarized thermal emission from Galactic dust with simulations of MHD turbulence

The Planck maps of polarized fraction (p) and polarization angle (ψ) contain information on the magnetic field structure. The data have been compared to synthetic polarized emission maps computed from simulations of anisotropic magnetohydrodynamical turbulence, assuming simply a uniform intrinsic polarization fraction of dust grains. The turbulent structure of the magnetic field is able to reproduce the main statistical properties of p and ψ that are observed directly in a variety of nearby clouds (dense cores excluded). The large-scale field orientation with respect to the line of sight plays a major role in the quantitative analysis of these statistical properties.

This study suggests that the large scatter of p at NH smaller than about 1022 cm-2 is due mainly to fluctuations in the magnetic field orientation along the line of sight, rather than to changes in grain shape and/or the efficiency of grain alignment.

Planck intermediate results. XX. Comparison of polarized thermal emission from Galactic dust with simulations of MHD turbulence

Two-dimensional distribution functions of polarization fraction p and column density NH in selected fields.

Comparison of polarized thermal emission from Galactic dust at 353 GHz with interstellar polarization in the visible

Correlation of polarized intensity in emission (MJy sr-1) with the degree of interstellar polarization.

The dust grains that emit the radiation seen by Planck in the submillimetre also extinguish and polarize starlight in the visible. Comparison of the polarization of the emission and of the interstellar polarization on selected lines of sight probed by stars provides unique new diagnostics of the emission and light scattering properties of dust.

Using ancillary catalogues of interstellar polarization and extinction of starlight, we obtain the degree of polarization, pV , and the optical depth in the V band to the star, τV. Toward these stars we measure the submillimetre polarized intensity, PS, and total intensity, IS, in the Planck 353 GHz channel. For those lines of sight through the diffuse interstellar medium with comparable values of the estimated column density and polarization directions close to orthogonal, we correlate properties in the submillimetre and visible to find two ratios, RS/V=(PS/IS)/(pVV) and RP/p=PS/pV , the latter focusing directly on the polarization properties of the aligned grain population alone.

We find RS/V = 4.2, with statistical and systematic uncertainties 0.2 and 0.3, respectively, and RP/p = 5.4 MJy sr-1, with uncertainties 0.2 and 0.3 MJy sr-1, respectively. Our estimate of RS/V is compatible with predictions based on a range of polarizing dust models that have been developed for the diffuse interstellar medium. However, our estimate of RP/p is not compatible with predictions, which are too low by a factor of about 2.5. This more discriminating diagnostic, RP/p, indicates that changes to the optical properties in the models of the aligned grain population are required.

Planck intermediate results. XXI. Comparison of polarized thermal emission from Galactic dust at 353 GHz with interstellar polarization in the visible


An overview of the polarized thermal emission from Galactic dust

This paper presents an overview of the polarized sky as seen by Planck HFI at 353 GHz, which is the most sensitive Planck channel for dust polarization. We construct and analyse maps of dust polarization fraction and polarization angle at 1 degree resolution, taking into account noise bias and possible systematic effects. The sensitivity of the Planck HFI polarization measurements allows for the first time a mapping of Galactic dust polarized emission on large scales, including low column density regions. We find that the maximum observed dust polarization fraction is high (pmax = 19.8%), in particular in some regions of moderate hydrogen column density (NH less than 2 x 1021 cm-2). The polarization fraction displays a large scatter at NH below a few 1021 cm-2. There is a general decrease in the dust polarization fraction with increasing column density above NH = 1x1021 cm-2 and in particular a sharp drop above NH = 1.5x1022 cm-2.

We characterize the spatial structure of the polarization angle using the angle dispersion function. We find that the polarization angle is ordered over extended areas of several square degrees, separated by filamentary structures of high angle dispersion function. These appear as interfaces where the sky projection of the magnetic field changes abruptly without variations in the column density. The polarization fraction is found to be anti-correlated with the dispersion of polarization angles. These results suggest that, at the resolution of 1 degree, depolarization is due mainly to fluctuations in the magnetic field orientation along the line of sight, rather than to the loss of grain alignment in shielded regions. We also compare the polarization of thermal dust emission with that of synchrotron measured with Planck, low-frequency radio data, and Faraday rotation measurements toward extragalactic sources. These components bear resemblance along the Galactic plane and in some regions such as the Fan and North Polar Spur regions. The poor match observed in other regions shows, however, that dust, cosmic-ray electrons, and thermal electrons generally sample different parts of the line of sight.

Planck intermediate results. XIX. An overview of the polarized thermal emission from Galactic dust

Distribution of the polarization fraction (p) as a function of gas column density over the whole sky.

 
Webpage curated by Juan D. Soler on behalf of the MISTIC team and the Planck collaboration. Contact: jsolerpu [at] ias.u-psud.fr