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

Nature Astronomy volume 7, pages 1295–1300 (2023)Cite this article

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Abstract

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 process1,2,3. Faraday rotation measures along many different sightlines across the Galaxy are a primary means to infer the magnetic field topology and strength from observations4,5,6,7. 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 map8,9,10. 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 medium11,12, and perform detailed polarized radiative transfer calculations on the resulting model13. 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 Bubble14 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.

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Fig. 1: All-sky Faraday RM map of the Milky Way and of a model galaxy from the Auriga suite of cosmological simulations.
Fig. 2: Magnitude of the Faraday RM along the Galactic longitude and latitude.
Fig. 3: Multipole spectrum of RM maps for ten different observer positions compared to observational data for the Milky Way.

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Data availability

No data available.

Code availability

Cluster properties and ionization are calculated with the WARPFIELD code11,42 and the spectral synthesis code CLOUDY v.17.00 (http://www.nublado.org/), respectively. Cosmological simulations are performed by the moving mesh code AREPO30 (https://arepo-code.org/wp-content/userguide/index.html) and for the radiative transfer postprocessing we use the radiative transfer code POLARIS25 (https://portia.astrophysik.uni-kiel.de/polaris/). We used Python and its associated libraries including Astropy, NumPy and Matplotlib for data analysis and presentation.

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Acknowledgements

S.R., R.S.K., E.W.P. and D.R. acknowledge support from the Deutsche Forschungsgemeinschaft in the Collaborative Research Center (SFB 881, ID 138713538) ‘The Milky Way System’ (subprojects A1, B1, B2 and B8) and from the Heidelberg Cluster of Excellence (EXC 2181, ID 390900948) ‘STRUCTURES: A unifying approach to emergent phenomena in the physical world, mathematics, and complex data’, funded by the German Excellence Strategy. R.S.K. also expresses thanks for funding from the European Research Council in the ERC Synergy Grant ‘ECOGAL – Understanding our Galactic ecosystem: From the disk of the Milky Way to the formation sites of stars and planets’ (ID 855130). R.G. acknowledges support from an STFC Ernest Rutherford Fellowship (ST/W003643/1). F.A.G. acknowledges support from ANID FONDECYT Regular 1211370, the Max Planck Society through a Partner Group grant and ANID Basal Project FB210003. The project benefited from computing resources provided by the State of Baden-Württemberg through bwHPC and DFG through grant INST 35/1134-1 FUGG, and from the data storage facility SDS@hd supported through grant INST 35/1314-1 FUGG. The Heidelberg team also express thanks for computing time provided by the Leibniz Computing Center (LRZ) for project pr74nu.

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Authors and Affiliations

  1. Institut für Theoretische Astrophysik, Universität Heidelberg, Zentrum für Astronomie, Heidelberg, Germany

    Stefan Reissl, Ralf S. Klessen, Eric W. Pellegrini & Daniel Rahner

  2. Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Universität Heidelberg, Heidelberg, Germany

    Ralf S. Klessen

  3. Max-Planck-Institut für Astrophysik, Garching, Germany

    Rüdiger Pakmor & Volker Springel

  4. Astrophysics Research Institute, Liverpool John Moores University, Liverpool, UK

    Robert Grand

  5. Instituto de Investigación Multidisciplinar en Ciencia y Tecnología, Universidad de La Serena, La Serena, Chile

    Facundo Gómez

  6. Departamento de Física y Astronomía, Universidad de La Serena, La Serena, Chile

    Facundo Gómez

  7. Department of Physics and Astronomy ‘Augusto Righi’, University of Bologna, via Gobetti 93/2, University of Bologna, Bologna, Italy

    Federico Marinacci

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Contributions

S.R. has run all polarized radiative transfer calculations and has performed most of the analysis. The text was jointly written by S.R. and R.S.K. The WARPFIELD cloud-cluster evolution model was mostly contributed by E.W.P. and D.R. The Augiga-6 data and support with the data handling have been provided by R.P., R.G., F.G., F.M. and V.S.

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Correspondence to Stefan Reissl.

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Reissl, S., Klessen, R.S., Pellegrini, E.W. et al. A reproduction of the Milky Way’s Faraday rotation measure map in galaxy simulations from global to local scales. Nat Astron 7, 1295–1300 (2023). https://doi.org/10.1038/s41550-023-02053-2

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