A reproduction of the Milky Way’s Faraday rotation measure map in galaxy simulations from global to local scales

Magnetic fields are of critical importance for our understanding of the origin and long-term evolution of the Milky Way. This is due to their decisive role in the dynamical evolution of the interstellar medium and their influence on the star-formation process(1-3). Faraday rotation measures along many different sightlines across the Galaxy are a primary means to infer the magnetic field topology and strength from observations(4-7). However, the interpretation of the data has been hampered by the failure of previous attempts to explain the observations in theoretical models and to synthesize a realistic multiscale all-sky rotation measures map(8-10). We here utilize a cosmological magnetohydrodynamic simulation of the formation of the Milky Way, augment it with a new star-cluster population-synthesis model for a more realistic structure of the local interstellar medium(11,12), and perform detailed polarized radiative transfer calculations on the resulting model(13). This yields an accurate first-principles prediction of the Faraday sky as observed on Earth. The results reproduce the observations of the Galaxy not only on global scales but also on local scales of individual star-forming clouds. They also indicate that the Local Bubble(14) containing our Sun dominates the rotation measures signal over large regions of the sky. Modern cosmological magnetohydrodynamic simulations of the Milky Way's formation, combined with a plausible model for star formation, stellar feedback and the distribution of free electrons in the interstellar medium, explain the rotation measures observations remarkably well, and thus contribute to a better understanding of the origin of magnetic fields in our Galaxy.