Environment matters: stronger magnetic fields in satellite galaxies

Magnetic fields are ubiquitous in the universe and an important component of the interstellar medium. It is crucial to accurately model and understand their properties in different environments and across all mass ranges of galaxies to interpret observables related to magnetic fields correctly. However, the assessment of the role of magnetic fields in galaxy evolution is often hampered by limited numerical resolution in cosmological simulations, in particular for satellite galaxies. To this end, we study the magnetic fields in high-resolution cosmological zoom simulations of disk galaxies (with $M_{200}\approx10^{10}$ to $10^{13},\mathrm{M}_\odot$) and their satellites within the Auriga galaxy formation model including cosmic rays. We find significantly higher magnetic field strengths in satellite galaxies compared to isolated dwarfs with a similar mass or star-formation rate, in particular after they had their first close encounter with their host galaxy. These are stronger on average by factors of 2-8 when compared at the same total mass, with a large scatter, ranging up to factors of $\sim$15. While this result is ubiquitous and independent of resolution in the satellites that are past their first infall, there seems to be a wide range of amplification mechanisms acting together. Our result highlights the importance of considering the environment of dwarf galaxies when interpreting their magnetic field properties as well as related observables such as their gamma-ray and radio emission, the latter being particularly relevant for future observations such as with the SKA observatory.

💡 Research Summary

This paper investigates how a galaxy’s environment influences the strength of its magnetic field, focusing on satellite dwarf galaxies versus isolated dwarfs. Using the Auriga galaxy formation model with cosmic‑ray magnetohydrodynamics (CR‑MHD), the authors performed high‑resolution cosmological zoom‑in simulations of 13 host haloes spanning $M_{200}=10^{10}$–$10^{13},M_\odot$. Each host includes a population of star‑forming satellites; the simulations cover three mass‑resolution levels (gas particle masses of $8\times10^{2}$, $6\times10^{3}$, and $5\times10^{4},M_\odot$) to test numerical convergence. An initial seed magnetic field of $10^{-14}$ G is imposed, and cosmic rays are injected at 10 % of supernova energy with an anisotropic diffusion coefficient $\kappa=10^{28},\mathrm{cm^{2},s^{-1}}$ (some runs use $3\times10^{28}$) plus Alfvén‑wave loss terms.

Satellites are classified into two evolutionary stages: (i) first approach (still on first infall, before pericentric passage) and (ii) post‑first‑infall (≥300 Myr after the first pericentre). Magnetic field strength is quantified as the volume‑weighted average magnetic energy density within a 20 kpc sphere around each galaxy, while star‑formation rates (SFR) are averaged over the last 100 Myr.

Key findings:

1. Systematically stronger fields in satellites after first pericentre. Compared to isolated dwarfs of the same total mass ($10^{10}$–$5\times10^{11},M_\odot$) or the same SFR (2×10⁻³–9×10⁻¹ $M_\odot$ yr⁻¹), post‑infall satellites exhibit magnetic fields that are on average 2–8 times higher, with extreme cases up to a factor of ~15. In four logarithmic mass bins the average amplification factors are 7.9, 6.0, 2.7, and 2.4; in four SFR bins they are 8.7, 3.2, 2.2, and 1.8.

2. Dependence on pericentric distance. Satellites that pass closer to the host (smaller pericentre in units of $R_{200}$) show larger magnetic enhancements. During pericentric passage the field is temporarily boosted throughout the satellite, likely because the averaging sphere overlaps the magnetised circumgalactic medium (CGM) of the host. After the passage, the enhancement persists mainly in the central region (≤5 kpc).

3. Resolution and CR‑physics robustness. The trends are present across all resolution levels; the difference between level‑3 and level‑4 runs is <10 % in average field strength. Simulations without CRs, with lower CR injection efficiency, or with higher diffusion coefficients reproduce the same qualitative behaviour, indicating that the result is not driven by a specific CR prescription.

4. Physical amplification mechanisms. The authors argue that several processes act together:

   • Compression of gas as the satellite falls into the host’s potential, increasing magnetic flux density.
   • Magnetic draping where the host’s ambient field is swept and wrapped around the leading edge of the satellite.
   • Turbulent driving induced by shear and shocks at pericentre, which they quantify via second‑order velocity structure functions, finding driving scales of 0.5–1 kpc, consistent with small‑scale dynamo action.
   • Cosmic‑ray pressure gradients that can further compress gas and stir turbulence.

5. Observational implications. Stronger magnetic fields directly affect synchrotron emission and Faraday rotation measures, meaning that satellite dwarfs may appear brighter in radio than predicted by models that ignore environmental amplification. This also impacts gamma‑ray emission from CR protons, since CR transport and Alfvén‑wave cooling depend on field strength. Consequently, interpretations of upcoming SKA observations of dwarf galaxies must account for the host‑satellite environment.

In summary, the paper provides robust evidence that the environment—specifically the first close encounter with a massive host—significantly amplifies magnetic fields in dwarf satellite galaxies. The amplification is largely independent of numerical resolution and persists across different CR‑MHD implementations. These findings call for a revision of galaxy‑evolution models and observational analyses to include environmental magnetic‑field enhancement, especially in the era of high‑sensitivity radio surveys.