Evolution of galactic magnetic fields

Abstract

We study the cosmic evolution of the magnetic fields of a large sample of spiral galaxies in a cosmologically representative volume by employing a semi-analytic galaxy formation model and numerical dynamo solver in tandem. We start by deriving timeand radius-dependent galaxy properties using the galform galaxy formation model, which are then fed into the nonlinear mean-field dynamo equations. These are solved to give the large-scale (mean) field as a function of time and galactocentric radius for a thin disc, assuming axial symmetry. A simple prescription for the evolution of the small-scale (random) magnetic field component is also adopted. We find that, while most massive galaxies are predicted to have large-scale magnetic fields at redshift z = 0, a significant fraction of them is expected to contain negligible large-scale field. Our model indicates that, for most of the galaxies containing large-scale magnetic fields today, the mean-field dynamo becomes active at z < 3. Moreover, the typical magnetic field strength at any given galactic stellar mass is predicted to decline with time up until the present epoch, in agreement with our earlier results. We compute the radial profiles of pitch angle, and find broad agreement with observational data for nearby galaxies.

Magnetic Fields and Halos in Spiral Galaxies

“…Dwarf galaxies can already host fully coherent fields at z ≈ 1, while giant disk galaxies may not have reached fully coherent field pattern in the Universe's lifetime up to now [46]. Similar results were attained recently by a semi-analytic galaxy formation model about the evolution of galactic magnetic fields [47]. Figure 5. (Left) Evolution of magnetic field strength in dwarf galaxies (DG), Milky-Way-type galaxies (MW), and in giant disk galaxies (GD).…”

The Origin of Large-Scale Magnetic Fields in Low-Mass Galaxies

“…Hindson et al [19] detected radio emission at λ6 cm from 22 out of 40 nearby dwarf galaxies to find the average strength of the total magnetic field of 5-8 µG (comprising both random and possible large-scale parts) assuming energy equipartition between cosmic rays and magnetic fields. Rodrigues et al [12] find that 20-50% of low-mass galaxies might host detectable large-scale magnetic fields, in agreement with Beck and Wielebinski [20, see arXiv:1302.5663 for an up-to-date list of galaxies ].…”

Synthesizing Observations and Theory to Understand Galactic Magnetic Fields: Progress and Challenges

“…Second, 2-D global axisymmetric, 1-D global axisymmetric 'no-z', or even 0-D local in r axisymmetric 'no-z' mean-field models can be useful in predicting and explaining observed statistical relations between various properties of galaxies. For example, recent work by Rodrigues et al [129] uses, as input, semi-analytic galaxy formation models (which themselves use merger-trees from cosmological N-body simulations as input) to solve for the magnetic fields of ∼ 3 × 10 6 galaxies through cosmic time, using a 1-D global mean-field dynamo simulation for each evolving galaxy. Such "population synthesis" models can be valuable tools for generating synthetic statistical data sets.…”