TY - JOUR A1 - Rüdiger, Günther A1 - Schultz, Manfred T1 - Large-scale dynamo action of magnetized Taylor-Couette flows JF - Monthly notices of the Royal Astronomical Society N2 - A conducting Taylor-Couette flow with quasi-Keplerian rotation law containing a toroidal magnetic field serves as a mean-field dynamo model of the Tayler-Spruit type. The flows are unstable against non-axisymmetric perturbations which form electromotive forces defining a effect and eddy diffusivity. If both degenerated modes with m = +/- 1 are excited with the same power then the global a effect vanishes and a dynamo cannot work. It is shown, however, that the Tayler instability produces finite alpha effects if only an isolated mode is considered but this intrinsic helicity of the single-mode is too low for an alpha(2) dynamo. Moreover, an alpha Omega dynamo model with quasi-Keplerian rotation requires a minimum magnetic Reynolds number of rotation of Rm similar or equal to 2000 to work. Whether it really works depends on assumptions about the turbulence energy. For a steeper-than-quadratic dependence of the turbulence intensity on the magnetic field, however, dynamos are only excited if the resulting magnetic eddy diffusivity approximates its microscopic value, eta(T) similar or equal to eta. By basically lower or larger eddy diffusivities the dynamo instability is suppressed. KW - dynamo KW - instabilities KW - MHD KW - magnetic fields Y1 - 2020 U6 - https://doi.org/10.1093/mnras/staa293 SN - 0035-8711 SN - 1365-2966 VL - 493 IS - 1 SP - 1249 EP - 1260 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Pfrommer, Christoph A1 - Werhahn, Maria A1 - Pakmor, Rudiger A1 - Girichidis, Philipp A1 - Simpson, Christine M. T1 - Simulating radio synchrotron emission in star-forming galaxies BT - small-scale magnetic dynamo and the origin of the far-infrared-radio correlation JF - Monthly notices of the Royal Astronomical Society N2 - In star-forming galaxies, the far-infrared (FIR) and radio-continuum luminosities obey a tight empirical relation over a large range of star-formation rates (SFR). To understand the physics, we examine magnetohydrodynamic galaxy simulations, which follow the genesis of cosmic ray (CR) protons at supernovae and their advective and anisotropic diffusive transport. We show that gravitational collapse of the proto-galaxy generates a corrugated accretion shock, which injects turbulence and drives a small-scale magnetic dynamo. As the shock propagates outwards and the associated turbulence decays, the large velocity shear between the supersonically rotating cool disc with respect to the (partially) pressure-supported hot circumgalactic medium excites Kelvin-Helmholtz surface and body modes. Those interact non-linearly, inject additional turbulence and continuously drive multiple small-scale dynamos, which exponentially amplify weak seed magnetic fields. After saturation at small scales, they grow in scale to reach equipartition with thermal and CR energies in Milky Way-mass galaxies. In small galaxies, the magnetic energy saturates at the turbulent energy while it fails to reach equipartition with thermal and CR energies. We solve for steady-state spectra of CR protons, secondary electrons/positrons from hadronic CR-proton interactions with the interstellar medium, and primary shock-accelerated electrons at supernovae. The radio-synchrotron emission is dominated by primary electrons, irradiates the magnetized disc and bulge of our simulated Milky Way-mass galaxy and weakly traces bubble-shaped magnetically loaded outflows. Our star-forming and star-bursting galaxies with saturated magnetic fields match the global FIR-radio correlation (FRC) across four orders of magnitude. Its intrinsic scatter arises due to (i) different magnetic saturation levels that result from different seed magnetic fields, (ii) different radio synchrotron luminosities for different specific SFRs at fixed SFR, and (iii) a varying radio intensity with galactic inclination. In agreement with observations, several 100-pc-sized regions within star-forming galaxies also obey the FRC, while the centres of starbursts substantially exceed the FRC. KW - dynamo KW - MHD KW - methods: numerical KW - cosmic rays KW - galaxies: formation KW - radio KW - continuum: galaxies Y1 - 2022 U6 - https://doi.org/10.1093/mnras/stac1808 SN - 0035-8711 SN - 1365-2966 VL - 515 IS - 3 SP - 4229 EP - 4264 PB - Oxford Univ. Press CY - Oxford ER -