Transition from small-scale to large-scale dynamo in a supernova-driven, multiphase medium

Abstract

Magnetic fields are widely recognised as critical at many scales to galactic dynamics and structure, including multiphase pressure balance, dust processing, and star formation. Using imposed magnetic fields cannot reliably model the interstellar medium's (ISM) dynamical structure nor phase interactions. Dynamos must be modelled. ISM models exist of turbulent magnetic fields using small-scale dynamo (SSD). Others model the large-scale dynamo (LSD) organising magnetic fields at scale of the disc or spiral arms. Separately, neither can fully describe the galactic magnetic field dynamics nor topology. We model the LSD and SSD together at sufficient resolution to use the low explicit Lagrangian resistivity required. The galactic SSD saturates within 20 Myr. We show that the SSD is quite insensitive to the presence of an LSD and is even stronger in the presence of a large-scale shear flow. The LSD grows more slowly in the presence of SSD, saturating after 5 Gyr vs. 1--2 Gyr in studies where the SSD is weak or absent. The LSD primarily grows in warm gas in the galactic midplane. Saturation of the LSD occurs due to $\alpha$-quenching near the midplane as the growing mean field produces a magnetic $\alpha$ that opposes the kinetic $\alpha$. The magnetic energy in our models of the LSD shows slightly sublinear response to increasing resolution, indicating that we are converging towards the physical solution at 1 pc resolution. Clustering supernovae in OB associations increases the growth rates for both the SSD and the LSD, compared to a horizontally uniform supernova distribution.

33 pages, 18 figures, 4 tables

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• Magnetic fields and cosmic rays in M31. II. Strength and distribution of the magnetic field components
• Understanding the radio luminosity function of star-forming galaxies and its cosmological evolution
• Strong turbulence and magnetic coherent structures in the interstellar medium
• Galactic magnetic fields II. Applying the model to nearby galaxies
• Modeling Local Bubble analogs II: Synthetic Faraday rotation maps
• Non-parametric Bayesian reconstruction of Galactic magnetic fields using Information Field Theory: The inclusion of line-of-sight information in ultra-high energy cosmic ray backtracking
• Modelling cosmic-ray transport: magnetised versus unmagnetised motion in astrophysical magnetic turbulence