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Polarisation properties of Milky-Way-like galaxies

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By Samuel George 2352 days ago

Aims. We study the polarisation properties, magnetic field strength, and synchrotron emission scale-height of Milky-Way-like galaxies in comparison with other spiral galaxies.

Methods. We used our 3D-emission model of the Milky Way Galaxy for viewing the Milky Way from outside at various inclinations in the way that spiral galaxies are observed. We analysed these Milky Way maps with techniques used to obtain the strength of magnetic fields, rotation measures (RMs), and scale-heights of synchrotron emission from observations of resolved galaxies and compared the results with the Milky Way model parameter. We also simulated a large sample of unresolved Milky-Way-like galaxies to study their statistical polarisation properties.

Results. When seen edge-on, the synchrotron emission from the Milky Way has an exponential scale-height of about 0.74 kpc, which is much lower than the values obtained from previous models. We find that current analysis methods overestimate the scale-height of synchrotron emission of galaxies by about 10% at an inclination of 80° and about 40% at an inclination of 70° because of contamination from the disk. The observed RMs for face-on galaxies derived from high-frequency polarisation measurements approximate to the Faraday depths (FDs) when scaled by a factor of two. For edge-on galaxies, the observed RMs are indicative of the orientation of the large-scale magnetic field, but are not closely related with the FDs. Assuming energy equipartition between the magnetic field and particles for the Milky Way results in an average magnetic-field strength that is about twice as high as the intrinsic value for a K factor of 100. The number distribution of the integrated polarisation percentages of a large sample of unresolved Milky-Way-like galaxies peaks at about 4.2% at 4.8 GHz and at about 0.8% at 1.4 GHz. Integrated polarisation angles rotated by 90° align very well with the position angles of the major axes, implying that unresolved galaxies do not have intrinsic RMs.

Conclusions. Simulated maps of the Milky Way Galaxy when viewed from outside at various inclination angles are the basis for a comparison with spiral galaxy maps. They are also helpful to check the accuracy of analysing methods of spiral galaxy observations. Unresolved Milky-Way-like galaxies are ideal background sources to investigate intergalactic magnetic fields. However, at 1.4 GHz they are typically polarised below about 1% and hence a high instrumental purity of future facilities such as the SKA is required to observed them.