Towards a hybrid dynamo model for the Milky Way

Abstract

Context. Based on the rapidly increasing all-sky data of Faraday rotation measures and polarised synchrotron radiation, the Milky Way's magnetic field can now be modelled with an unprecedented level of detail and complexity. Aims. We aim to complement this phenomenological approach with a physically motivated, quantitative dynamo model -a model that moreover allows for the evolution of the system as a whole, instead of just solving the induction equation for a fixed static disc. Methods. Building on the framework of mean-field magnetohydrodynamics and extending it to the realm of a hybrid evolution, we performed three-dimensional global simulations of the Galactic disc. To eliminate free parameters, closure coefficients embodying the mean-field dynamo were calibrated against resolved local simulations of supernova-driven interstellar turbulence. Results. The emerging dynamo solutions comprise a mixture of the dominant axisymmetric S0 mode with even parity, and a subdominant A0 mode with odd parity. Notably, this superposition of modes creates a strong localised vertical field on one side of the Galactic disc. Moreover, we found significant radial pitch angles that decay with radius, which can be explained by flaring of the disc. In accordance with previous work, magnetic instabilities appear to be restricted to the calmer outer Galactic disc. Their main effect is to create strong fields at large radii such that the radial scale length of the magnetic field increases from 4 kpc (for a mean-field dynamo alone) to about 10 kpc in the hybrid models -the latter being in much better agreement with observations. Conclusions. There remain aspects (e.g., spiral arms, X-shaped halo fields, fluctuating fields) that are not captured by the current model and that will require further development towards a fully dynamical evolution. Nevertheless, we demonstrate that a hybrid modelling of the Galactic dynamo is feasible and can serve as a foundation for future efforts.

Three-dimensional structure of the magnetic field in the disk of the Milky Way

“…An offset between the axes of the Galaxy and its magnetic dipole would cause a vertical oscillation of azimuthal and radial field components about the mid-plane similar to that described by Gressel et al (2013). The Parker spiral provides a mathematical description that can easily fit our data (see Fig.…”

Magnetic fields in spiral galaxies

“…Major progress was also achieved in numerical modelling of the magnetized ISM (e.g. Avillez and Breitschwerdt 2005;Gissinger et al 2009;Hill et al 2012;Gressel et al 2013;Gent et al 2013;Machida et al 2013), using a variety of codes and assumptions. Not surprisingly, the results differ and are not always consistent with observations, which calls for further efforts on both sides.…”

Do magnetic fields influence gas rotation in galaxies?

“…Dynamo-excited magnetic fields never show such configurations (Gressel et al 2013). The Alfvén velocity is an upper limit for the change of the rotation by the magnetic force for the above defined profiles.…”

“…Mean-field αΩ dynamo models also show an exponential radial profile of the magnetic field, but mostly with shorter scale lengths of the order of 5 kpc for grand design spirals (Gressel et al 2013). In the cosmological simulations of Beck et al (2012), the field strength in protogalactic halos approximately follows a 1/r profile.…”