The small-scale dynamo in a multiphase supernova-driven medium

TL;DR

This study investigates how supernova-driven turbulence in a multiphase interstellar medium influences the growth of small-scale magnetic fields, revealing intermittent growth patterns and phase-dependent saturation levels.

Contribution

It provides a realistic analysis of SSD growth in a multiphase, supernova-driven ISM, highlighting the roles of hot gas, vorticity, and turbulence characteristics.

Findings

SSD growth is intermittent and phase-dependent.

Hot gas exhibits the fastest magnetic field growth.

Magnetic energy saturates below 5% of kinetic energy in hot gas.

Abstract

Magnetic fields grow quickly, even at early cosmological times, suggesting the action of a small-scale dynamo (SSD) in the interstellar medium (ISM) of galaxies. Many studies have focused on idealized, isotropic, homogeneous, turbulent driving of the SSD. Here we analyze more realistic simulations of supernova-driven turbulence to understand how it drives an SSD. We find that SSD growth rates are intermittently variable as a result of the evolving multiphase ISM structure. Rapid growth in the magnetic field typically occurs in hot gas, with the highest overall growth rates occurring when the fractional volume of hot gas is large. SSD growth rates correlate most strongly with vorticity and fluid Reynolds number, which also both correlate strongly with gas temperature. Rotational energy exceeds irrotational energy in all phases, but particularly in the hot phase while SSD growth is most rapid. Supernova (SN) rate does not significantly affect the ISM average kinetic energy density. Rather, higher temperatures associated with high SN rates tend to increase SSD growth rates. SSD saturates with total magnetic energy density around 5% of equipartition to kinetic energy density, increasing slightly with magnetic Prandtl number. While magnetic energy density in the hot gas can exceed that of the other phases when SSD grows most rapidly, it saturates below 5% of equipartition with kinetic energy in the hot gas, while in the cold gas it attains 100%. Fast, intermittent growth of the magnetic field appears to be a characteristic behavior of SN-driven, multiphase turbulence.