Evolution of magnetic fields in galaxies and future observational tests with the Square Kilometre Array

By T. Arshakian, Rainer Beck, M. Krause, D. Sokoloff. 2008

160 citations recorded by the source corpus.

Aims. In the context of models of galaxy formation and evolution, we investigate the cosmological evolution of large- and small-scale magnetic fields inside galaxies. Methods. We use the dynamo theory to derive the timescales of amplification and ordering of magnetic fields in disk and puffy galaxies. Turbulence in protogalactic halos generated by thermal virialization can drive an efficient turbulent dynamo. Results from simulations of hierarchical structure formation cosmology provide a tool to develop an evolutionary model of regular magnetic fields coupled with galaxy formation and evolution. Results. The turbulent (small-scale) dynamo was able to amplify a weak seed magnetic field in halos of protogalaxies to a few μG strength within a few 10 8 yr. This turbulent field served as a seed to the mean-field (large-scale) dynamo. Galaxies similar to the Milky Way formed their disks at z ≈ 10 and regular fields of μG strength and a few kpc coherence length were generated within 2 Gyr (at z ≈ 3), but field-ordering on the coherence scale of the galaxy size required an additional 6 Gyr (at z ≈ 0.5). Giant galaxies formed their disks at z ≈ 10, allowing more efficient dynamo generation of strong regular fields (with kpc coherence length) already at z ≈ 4. However, the age of the Universe is short for fully coherent fields in giant galaxies larger than 15 kpc to have been achieved. Dwarf galaxies should have hosted fully coherent fields at z ≈ 1. After a major merger, the strength of the turbulent field is enhanced by a factor of a few. Conclusions. This evolutionary scenario can be tested by measurements of polarized synchrotron emission and Faraday rotation with the planned Square Kilometre Array (SKA). We predict an anticorrelation between galaxy size and ratio between ordering scale and galaxy size. Weak regular fields (small Faraday rotation) in galaxies at z < 3 are signatures of major mergers. Undisturbed dwarf galaxies should host fully coherent fields, giving rise to strong Faraday rotation signals. Radio observations may serve as a clock for measuring the time since the last major merger.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/22c3163e3cfdbf2c30efe675ff02564921337714

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Citations in the corpus

THE GENERATION OF STRONG MAGNETIC FIELDS DURING THE FORMATION OF THE FIRST STARS (2010, background): “The importance of the small-scale dynamo during galaxy formation was previously suggested by Beck et al. (1994), Arshakian et al. (2009), and de Souza & Opher (2010).”
Small-scale dynamo action during the formation of the first stars and galaxies - I. The ideal MHD limit (2010, methodology): “The small-scale dynamo was introduced by Kazantsev (1968) in an analytic framework, which was improved subsequently in various works (e.g. Subramanian 1998, 1999; Brandenburg & Subramanian 2005; Arshakian et al. 2009).”
Magnetic Fields in the Large-Scale Structure of the Universe (2011, background): “Such turbulence may create strong magnetic fields in the first galaxies and around them (Ryu et al. 2008, Arshakian et al. 2009, de Souza and Opher 2010, Schleicher et al. 2010).”
A NEW JEANS RESOLUTION CRITERION FOR (M)HD SIMULATIONS OF SELF-GRAVITATING GAS: APPLICATION TO MAGNETIC FIELD AMPLIFICATION BY GRAVITY-DRIVEN TURBULENCE (2011, background): “3 (Bernet et al. 2008; Arshakian et al. 2009, and references therein), emphasizing the significance of the dynamo action even in the early stages of galaxy evolution and star formation.”
Simulations of magnetic fields in isolated disc galaxies (2012, methodology): “A further amplification of these fields can then proceed through structure formation flows (e.g. Dolag et al. 1999), a Galactic Dynamo (e.g. Hanasz et al. 2004), or turbulent amplification (e.g. Arshakian et al. 2009).”