# Decaying turbulence and magnetic fields in galaxy clusters

## TL;DR

This study investigates how turbulence and magnetic fields decay in galaxy clusters after major mergers, revealing power-law decay behaviors influenced by flow conditions and providing insights into magnetic field evolution in astrophysical environments.

## Contribution

The paper presents detailed numerical simulations of turbulence and magnetic field decay in galaxy clusters, highlighting new decay scalings in transonic regimes and implications for magnetic field evolution.

## Key findings

- Velocity and magnetic fields decay as power laws over time.
- Decay exponents depend on flow compressibility and regime.
- Faraday rotation measure also decays as a power law, steeper than previous models.

## Abstract

We explore the decay of turbulence and magnetic fields generated by fluctuation dynamo action in the context of galaxy clusters where such a decaying phase can occur in the aftermath of a major merger event. Using idealized numerical simulations that start from a kinetically dominated regime we focus on the decay of the steady state rms velocity and the magnetic field for a wide range of conditions that include varying the compressibility of the flow, the forcing wave number, and the magnetic Prandtl number. Irrespective of the compressibility of the flow, both the rms velocity and the rms magnetic field decay as a power-law in time. In the subsonic case we find that the exponent of the power-law is consistent with the $-3/5$ scaling reported in previous studies. However, in the transonic regime both the rms velocity and the magnetic field initially undergo rapid decay with an $\approx t^{-1.1}$ scaling with time. This is followed by a phase of slow decay where the decay of the rms velocity exhibits an $\approx -3/5$ scaling in time, while the rms magnetic field scales as $\approx -5/7$. Furthermore, analysis of the Faraday rotation measure reveals that the Faraday RM decays also decays as a power law in time $\approx t^{-5/7}$; steeper than the $\sim t^{-2/5}$ scaling obtained in previous simulations of magnetic field decay in subsonic turbulence. Apart from galaxy clusters, our work can have potential implications in the study of magnetic fields in elliptical galaxies.