The Turbulent Dynamo in Highly Compressible Supersonic Plasmas
  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.
