Highlighting the turbulent Galactic magnetic fields

Magnetic fields are omnipresent in the Milky Way and play a crucial role in many physical processes in the interstellar medium (ISM).

JFR Figure 1

Because interstellar matter is never completely neutral, magnetic fields are locked into a diffuse plasma and have a significant influence on the distribution of matter through the ISM. The interaction between the magnetic field lines and the gas induces an additional pressure in the ISM and can control, among other important physical processes, the star formation process in the Galaxy. Unfortunately, magnetic fields cannot be observed directly. Since we know that they are acting on a broad range of spatial scales in the Galaxy, we need a tracer which allows us to connect the large-scale field, which is believed to be regular and following the morphology of Galactic arms, with the small-scale field, which is believed to be affected by turbulence and quasirandom. To study the Galactic fluctuations in the magnetic fields, Jean-François Robitaille, member of Anna Scaife’s LODESTONE project at the Jodrell Bank Centre for Astrophysics, and collaborators from the Mullard Space Science Laboratory and the S-band Polarization All Sky Survey (S-PASS), analysed the southern hemisphere map of the polarised synchrotron emission at 2.3 GHz observed by the Australian telescope Parkes.

The analysis is based on two decomposition techniques allowing them to get a new perspective on the magnetic field fluctuations at multiple angular scales in the Milky Way. The first technique is the polarisation gradient, initially developed to analyse the small scale turbulent fluctuations in the Magneto-Ionic Medium (MIM) and the second is the spin-2 decomposition in two opposite parities commonly called the E- and B-mode. The latter is mainly used for polarisation analysis of the cosmic microwave background. However, the authors show that particular alignments of the polarisation vectors over elongated structures in the MIM can produce an asymmetry between the E- and B-mode polarisation. The combination of both decomposition techniques using a wavelet-based formalism allowed the authors to locate the position of the E- or B-mode features responsible for the local asymmetries between the two polarisation types. In analysed subregions, the perturbations of the magnetic field are trigged by star clusters associated with HII regions, a Galactic superbubble and large-scale outflows at low Galactic latitude. These perturbations could contribute to the energy injection at different spatial scales in the turbulent ISM. This new work has recently been accepted for publication by Monthly Notices of the Astronomical Royal Society.

Reference Robitaille et al. 2017, MNRAS

JFR Figure 1

Figure 1: The giant magnetised outflows from the Galactic plane, known as the Fermi bubbles, identified in the top panel through the S-PASS polarisation intensity map and in the bottom panel through the polarisation gradient applied at large angular scale (~65 degrees).


JFR Figure 2

Figure 2 : Comparison between the polarisation gradient map (left) at 200 arcmin and the Bmode map (right) seen from the southern Galactic pole, both overlaid with the gradient of polarisation maxima chains.

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