Polarisation properties of Milky-Way-like galaxies

TL;DR

This study models the polarisation and magnetic properties of Milky-Way-like galaxies, revealing insights into their synchrotron emission scale-height, magnetic field strength, and polarisation characteristics from an external perspective.

Contribution

It provides a 3D emission model of the Milky Way viewed as an external galaxy, improving estimates of synchrotron scale-height and magnetic field strength, and analyzing polarisation properties.

Findings

Synchrotron emission scale-height is about 0.74 kpc when viewed edge-on.

Current methods overestimate synchrotron scale-height by 10-40% depending on inclination.

Polarisation angles align with galaxy major axes, indicating no intrinsic RMs in unresolved galaxies.

Abstract

(Abridged) We study the polarisation properties, magnetic field strength, and synchrotron emission scale-height of Milky-Way-like galaxies in comparison with other spiral galaxies. We use our 3D-emission model of the Milky Way Galaxy for viewing the Milky Way from outside at various inclinations as spiral galaxies are observed. When seen edge-on the synchrotron emission from the Milky Way has an exponential scale-height of about 0.74 kpc, which is much smaller 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 degree and about 40% at an inclination of 70 degree 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 well related with the FDs. Assuming energy equipartition between the magnetic field and particles for the Milky Way results in an average magnetic-field strength, which is about two times larger than 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 1.4GHz. Integrated polarisation angles rotated by 90 degree align very well with the position angles of the major axes, implying that unresolved galaxies do not have intrinsic RMs.