RSAA Colloquia / Seminars / Feast-of-Facts: Thursday, 26 March 2015, 11:00-12:00; Duffield Lecture Theatre

Jennifer Schober

"The Origin of Strong Magnetic Fields in Galaxies"

Local galaxies are in a state of ’energy equipartition’ which means that the kinetic energy density equals roughly the thermal one and the magnetic one. Thus, we can deduce that magnetic fields play a crucial role in galactic processes like for example star formation. In models of young galaxies magnetic fields are usually not considered as they are assumed not to be dynamical important at high redshifts. In the presence of turbulence, however, the ’turbulent dynamo’ can amplify weak magnetic seed fields by randomly stretching, twisting, and folding the field lines. The details of this process depend on the nature of turbulence, i.e. on the hydrodynamic and magnetic Reynolds numbers, and on the compressibility of the gas. In my talk I will discuss the physics of the turbulent dynamo and sketch the analytical derivation of its properties. I compare these theoretical results with the ones found in state-of-the-art numerical simulations. As an application I discuss the magnetic field amplification in a young galaxy, where turbulence is driven by accretion and supernova explosions. In our semi-analytical model we follow the exponential growth of the magnetic field on the dissipative scale and also the subsequent transport of the magnetic energy to larger scales in the non-linear dynamo phase. Depending on the parameters of our model we find that equipartition of magnetic and kinetic energy, i.e. a field strength of roughly 10^(-5) G, is reached within 4 to 270 Myr. Thus, we expect that the turbulent dynamo can generate strong unordered fields already in very young galaxies.