Magnetic fields are dynamically important in the ISM, but their influence on star formation is still a matter of active research, requiring more and better data. Observations of magnetic fields, however, are necessarily indirect and the translation from observed quantities to magnetic field strength or direction is not straightforward. Here, stellar feedback plays a crucial role by creating ionised regions, thus determining the properties of Faraday rotation measure observations.
In this work, we use magnetohydrodynamic simulations of the interstellar medium in star-forming disk galaxies that are part of the SILCC project. These simulations span environments from our quiescent local galactic neighbourhood to violent starburst galaxies. We investigate the spatial magnetic field structure and discuss how accurately fields can be reconstructed from observations of Faraday rotation .
To this end, we quantify how well the magnetic field is ordered by calculating two measures of its spatial coherence: Besides the autocorrelation length, we introduce the patch length as the length over which the line-of-sight magnetic field retains its sign. Both methods indicate a correlation length of a few tens of parsec, showing that Faraday tomography averaging over longer sightlines misrepresents the magnetic field. We find that the correlation length is weakly anti-correlated with turbulence induced by stellar feedback in disk galaxies.
We compute the rotation measure and find that its main contribution stems from the warm ionised and neutral media. Correlating the rotation measure with the average line-of-sight magnetic field, we provide, for the first time, conversion factors to obtain the true field strength from Faraday rotation observations. Taken together, our research contributes to reconstructing the magnetic field from data and might guide future observations.
