Magnetic field evolution and reversals in spiral galaxies (article)
Bate, Matthew R,
Monthly Notices of the Royal Astronomical Society
Oxford University Press (OUP)
© 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society
We study the evolution of galactic magnetic fields using 3D smoothed particle magnetohydrodynamics (SPMHD) simulations of galaxies with an imposed spiral potential.We consider the appearance of reversals of the field, and amplification of the field. We find that magnetic field reversals occur when the velocity jump across the spiral shock is above ≈20 km s-1, occurring where the velocity change is highest, typically at the inner Lindblad resonance in our models. Reversals also occur at corotation, where the direction of the velocity field reverses in the corotating frame of a spiral arm. They occur earlier with a stronger amplitude spiral potential, and later or not at all with weaker or no spiral arms. The presence of a reversal at radii of around 4-6 kpc in our fiducial model is consistent with a reversal identified in the MilkyWay, though we caution that alternative Galaxy models could give a similar reversal. We find that relatively high resolution, a few million particles in SPMHD, is required to produce consistent behaviour of the magnetic field. Amplification of the magnetic field occurs in the models, and while some may be genuinely attributable to differential rotation or spiral arms, some may be a numerical artefact.We check our results using ATHENA, finding reversals but less amplification of the field, suggesting that some of the amplification of the field with SPMHD is numerical.
This work used the DiRAC Complexity system, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility (www.dirac.ac.uk ). This equipment is funded by BIS National E-Infrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1. DiRAC is part of the National E-Infrastructure. This work also used the University of Exeter Supercomputer, a DiRAC Facility jointly funded by STFC, the Large Facilities Capital Fund of BIS, and the University of Exeter. CLD acknowledges support by the European Research Council under the European Community’s Seventh Framework Programme (FP7/2011- 2016 grant agreement no. 280104, LOCALSTAR). DJP is funded by a Future Fellowship (FT130100034) from the Australian Research Council (ARC). MRB and TST acknowledge support by the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013 grant agreement no. 339248)
This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.
The dataset associated with this article is in ORE at http://hdl.handle.net/10871/28584
Vol. 461, pp. 4482 - 4495