The Turbulent Dynamo in Highly Compressible Supersonic Plasmas
Federrath, C.; Schober, J.; Bovino, S.; Schleicher, D. R. G., 2014, The Astrophysical Journal Letters, in press [ PDF ]
The turbulent dynamo may explain the origin of cosmic magnetism. While the exponential amplification of magnetic fields has been studied for incompressible gases, little is known about dynamo action in highly-compressible, supersonic plasmas, such as the interstellar medium of galaxies and the early Universe. Here we perform the first quantitative comparison of theoretical models of the dynamo growth rate and saturation level with three-dimensional magnetohydrodynamical simulations of supersonic turbulence with grid resolutions of up to 1024^3 cells. We obtain numerical convergence and find that dynamo action occurs for both low and high magnetic Prandtl numbers Pm = nu/eta = 0.1-10 (the ratio of viscous to magnetic dissipation), which had so far only been seen for Pm >= 1 in supersonic turbulence. We measure the critical magnetic Reynolds number, Rm_crit = 129 (+43, -31), showing that the compressible dynamo is almost as efficient as in incompressible gas. Considering the physical conditions of the present and early Universe, we conclude that magnetic fields need to be taken into account during structure formation from the early to the present cosmic ages, because they suppress gas fragmentation and drive powerful jets and outflows, both greatly affecting the initial mass function of stars.
The movie shows the magnetic energy growth for two extremely different magnetic Prandtl numbers, Pm=0.1 (left-hand panel) and Pm=10 (right-hand panel). The magnetic field grows faster in high-Pm plasmas, such as in the interstellar medium and in the early Universe, when the first stars formed. Note that the field grows exponentially by more than 8 orders of magnitude before it reaches saturation.
We thank R. Banerjee and R. Klessen for stimulating discussions on the turbulent dynamo and the anonymous referee for their useful comments. C.F. acknowledges funding provided by the Australian Research Council's Discovery Projects (grants DP110102191, DP130102078, DP150104329). D.R.G.S., J.S. and S.B. thank for funding via the DFG priority program 1573 'The Physics of the Interstellar Medium' (grants SCHL 1964/1-1, BO 4113/1-2). We gratefully acknowledge the Jülich Supercomputing Centre (grant hhd20), the Leibniz Rechenzentrum and the Gauss Centre for Supercomputing (grant pr32lo), the Partnership for Advanced Computing in Europe (PRACE grant pr89mu), and the Australian National Computing Infrastructure (grant ek9). The software used in this work was in part developed by the DOE-supported Flash Center for Computational Science at the University of Chicago.