Magnetised winds in dwarf galaxies

¹ Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX13, RH, UK
e-mail: yohan.dubois@physics.ox.ac.uk
² Universität Zürich, Institute für Theoretische Physik, Winterthurerstrasse 190, 8057 Zürich, Switzerland
³ CEA Saclay, DSM/IRFU/SAP, Bâtiment 709, 91191 Gif-sur-Yvette Cedex, France

Received: 29 July 2009
Accepted: 7 July 2010

Abstract

Context. The generation and the amplification of magnetic fields in the current cosmological paradigm are still open questions. The standard theory is based on an early field generation by Biermann battery effects, possibly at the epoch of reionisation, followed by a long period of field amplification by galactic dynamos. The origin and the magnitude of the inter-galactic magnetic field is of primordial importance in this global picture, as it is considered to be the missing link between galactic magnetic fields and cluster magnetic fields on much larger scales.

Aims. We are testing whether dwarf galaxies are good candidates to explain the enrichment of the Inter-Galactic Medium (IGM): after their discs form and trigger galactic dynamos, supernova feedback will launch strong winds, expelling magnetic field lines in the IGM.

Methods. We have performed magneto-hydrodynamics simulations of an isolated dwarf galaxy, which forms self-consistently inside a cooling halo. Using the RAMSES code, we simulated for the first time the formation of a magnetised supernova-driven galactic outflow. This simulation is an important step towards a more realistic modelling using fully cosmological simulations.

Results. Our simulations reproduce well the observed properties of magnetic fields in spiral galaxies. The formation and the evolution of our simulated disc leads to a strong magnetic field amplification: the magnetic field in the final wind bubble is one order of magnitude larger than the initial value. The magnetic field in the disc, essentially toroidal, is growing linearly with time as a consequence of differential rotation.

Conclusions. We discuss the consequence of this simple mechanism on the cosmic evolution of the magnetic field: we propose a new scenario for the evolution of the magnetic field, with dwarf galaxies playing a key role in amplifying and ejecting magnetic energy in the IGM, resulting in what we call a “cosmic dynamo” that could contribute to the very high field strengths observed in galaxies and clusters today.