Rotation measure and synchrotron emission signatures in simulations of magnetized galactic discs

TL;DR

This study investigates how different models of electron density affect Faraday rotation measures and examines the evolution of synchrotron emission in simulated galactic discs, revealing the importance of accurate electron distribution for magnetic field analysis.

Contribution

It provides a detailed analysis of magnetic field signatures in galaxy simulations, highlighting the sensitivity of Faraday RM to electron density models and the evolution of synchrotron luminosity.

Findings

RM profiles are highly sensitive to electron density models.

Synchrotron luminosity decreases over time with magnetic energy decay.

Equipartition assumptions may lead to incorrect interpretations of magnetic and cosmic ray energies.

Abstract

We analyse observational signatures of magnetic fields for simulations of a Milky-Way like disc with supernova-driven interstellar turbulence and self-consistent chemical processes. In particular, we post-process two simulations data sets of the SILCC Project for two initial amplitudes of the magnetic field, $B_0 = $ 3 and 6 $\mu$G, to study the evolution of Faraday rotation measures (RM) and synchrotron luminosity. For calculating the RM, three different models of the electron density $n_e$ are considered. A constant electron density, and two estimations based on the density of ionized species and the fraction of the total gas, respectively. Our results show that the RM profiles are extremely sensitive to the $n_e$ models, which assesses the importance of accurate electron distribution observations/estimations for the magnetic fields to be probed using Faraday RMs. As a second observable of the magnetic field, we estimate the synchrotron luminosity in the simulations using a semi-analytical cosmic ray model. We find that the synchrotron luminosity decreases over time, which is connected to the decay of magnetic energy in the simulations. The ratios between the magnetic, the cosmic ray, and the thermal energy density indicate that the assumption of equipartition does not hold for most regions of the ISM. In particular, for the ratio of the cosmic ray to the magnetic energy the assumption of equipatition could lead to a wrong interpretation of the observed synchrotron emission.