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Abstract: Aims. We investigate the cosmological evolution of large- and small-scalemagnetic fields in galaxies in the light of present models of formation andevolution of galaxies. Methods. We use the dynamo theory to derive thetimescales of amplification and ordering of magnetic fields in disk and puffygalaxies. Turbulence in protogalactic halos generated by thermal virializationcan drive an efficient turbulent dynamo. Results from simulations ofhierarchical structure formation cosmology provide a tool to develop anevolutionary model of regular magnetic fields coupled with galaxy formation andevolution. Results. The turbulent small-scale dynamo was able to amplify aweak seed magnetic field in halos of protogalaxies to a few muG strength withina few 10^8 yr. This turbulent field served as a seed to the mean-fieldlarge-scale dynamo. Galaxies similar to the Milky Way formed their disks atz~10 and regular fields of muG strength and a few kpc coherence length weregenerated within 2 Gyr at z~3, but field-ordering on the coherence scale ofthe galaxy size required an additional 6 Gyr at z~0.5. Giant galaxies formedtheir disks at z~10, allowing more efficient dynamo generation of strongregular fields with kpc coherence length already at z~4. However, the age ofthe Universe is short for fully coherent fields in giant galaxies larger than15 kpc to have been achieved. Dwarf galaxies should have hosted fully coherentfields at z~1. After a major merger, the strength of the turbulent field isenhanced by a factor of a few. Conclusions. This evolutionary scenario can betested by measurements of polarized synchrotron emission and Faraday rotationwith the planned SKA. We predict an anticorrelation between galaxy size andratio between ordering scale and galaxy size abridged.

Author: Tigran G. Arshakian, Rainer Beck, Marita Krause, Dmitry Sokoloff

Source: https://arxiv.org/

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