Galactic winds and the origin of large-scale magnetic fields

Abstract

Context. Observations of dwarf galaxies suggest the presence of large-scale magnetic fields. However the size and slow rotation of these galaxies appear insufficient to support a mean-field dynamo action to excite such fields. Aims. Here we suggest a new mechanism to explain large-scale magnetic fields in galaxies that are too small to support mean-field dynamo action. The key idea is that we do not identify large-scale and mean magnetic fields. In our scenario the the magnetic structures originate from a small-scale dynamo which produces small-scale magnetic field in the galactic disc and a galactic wind that transports this field into the galactic halo where the large turbulent diffusion increases the scale and order of the field. As a result, the magnetic field becomes large-scale; however its mean value remains vanishing in a strict sense. Methods. We verify the idea by numerical modelling of two distinct simplified configurations, a thin disc model using the no-z approximation, and an axisymmetric model using cylindrical r, z coordinates. Results. Each of these allows reduction of the problem to two spatial dimensions. Taken together, the models support the proposition that the general trends will persist in a fully 3D model. We demonstrate that a pronounced large-scale pattern can develop in the galactic halo for a wide choice of the dynamo governing parameters. Conclusions. We believe that our mechanism can be relevant to explaining the presence of the fields observed in the halos of dwarf galaxies. We emphasize that detailed modelling of the proposed scenario needs 3D simulations, and adjustment to the specific dynamo governing parameters of dwarf galaxies.

Impact of Cosmic-Ray Transport on Galactic Winds

“…Galactic winds are observed ubiquitously in starforming galaxies and significantly affect their chemical and dynamical evolution (Veilleux, Cecil, & Bland-Hawthorn 2005). Galactic winds redistribute angular momentum, aiding in the formation of extended disks (Brook et al 2011;Übler et al 2014), help to produce large-scale magnetic fields in dwarf galaxies (Moss & Sokoloff 2017), and pollute the intergalactic medium with metals (Steidel et al 2010;Booth et al 2012). Additionally, most galaxies are missing a large fraction of baryons compared to the cosmological average (Bell et al 2003).…”

Chang-Es

“…The complex behavior of galactic dynamos is not fully understood yet, and the influence of galactic outflows in particular is still a topic of discussion. Outflows can order a turbulentdisk magnetic field such that it evolves into a regular halo field in dwarf galaxies (Moss & Sokoloff 2017). In Milky Way-type spiral galaxies, galactic outflows lead to two countervailing effects: They are crucial to remove small-scale helicity and hence to avoid quenching of dynamo action, but also lead to field losses into the halo that are responsible for the saturation of the dynamo (e.g., Bendre et al 2015;Chamandy et al 2015).…”

Investigation of the cosmic ray population and magnetic field strength in the halo of NGC 891

“…A turbulent field may be transported together with the gas in an outflow while an ordered field can support or suppress the outflow, depending on its strength and orientation. This is complicated further due to the interplay of the magnetic field with an inhomogeneous outflow in which isotropic turbulent fields are converted into anisotropic turbulent fields due to shear and compression, thus creating an ordered field in the halo (Elstner et al 1995;Moss & Sokoloff 2017).…”