The dynamical impact of cosmic rays in the Rhea magnetohydrodynamic simulations

¹ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
² Centre de Recherche Astrophysique de Lyon UMR5574, ENS de Lyon, Univ. Lyon1, CNRS, Université de Lyon, 69007 Lyon, France
³ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁵ Elizabeth S. and Richard M. Cashin Fellow at the Radcliffe Institute for Advanced Studies at Harvard University, 10 Garden Street, Cambridge, MA 02138, USA
⁶ Institute of Physics, Laboratory for Galaxy Evolution and Spectral Modelling, EPFL, Observatoire de Sauverny, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁷ Istituto di Astrofisica e Planetologia Spaziali (IAPS), INAF, Via Fosso del Cavaliere 100, 00133 Roma, Italy
⁸ Leibniz-Institut für Astrophysik Potsdam (AIP), An der, Sternwarte 16, D-14482 Potsdam, Germany
⁹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
¹⁰ SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY169SS, UK
¹¹ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, 40122 Bologna, Italy

^⋆ Corresponding author.

Received: 14 January 2025
Accepted: 6 June 2025

Abstract

This study explores the dynamical impact of cosmic rays (CRs) in Milky Way-like galaxies using the Rhea simulation suite. Cosmic rays, with their substantial energy density, influence the interstellar medium (ISM) by supporting galactic winds, modulating star formation, and shaping ISM energetics. The simulations incorporate a multiphase ISM, self-consistent CR transport in the advection-diffusion approximation, and interactions with magnetic fields to study their effects on galaxy evolution. Key findings reveal that CRs reduce star formation rates (SFRs) and drive weak, but sustained outflows with mass-loading factors of ∼0.2, transporting a substantial fraction (20%−60%) of the injected CR energy. These CR-driven outflows are launched not just from the galactic center, but across the entire disk, illustrating their pervasive dynamical influence. Galactic disks supported by CRs exhibit broader vertical structures compared to magnetic-field-dominated setups, although the scale heights are similar. CR feedback enhances magnetic flux transport to the circumgalactic medium (CGM), leading to a magnetically enriched CGM with field strengths of ∼0.5μG, while reducing gas temperatures to ≲10⁵ K. The CR energy is relatively smoothly distributed in the disk, with gradient lengths exceeding the typical size of molecular clouds, indicating that the CR behavior is not adiabatic.