Decaying turbulence and magnetic fields in galaxy clusters

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.