Origin of strong magnetic fields in Milky Way-like galactic haloes

Abstract

An analytical model predicting the growth rates, the absolute growth times and the saturation values of the magnetic field strength within galactic haloes is presented. The analytical results are compared to cosmological magnetohydrodynamics (MHD) simulations of Milky Way‐like galactic halo formation performed with the N‐body/spmhd code gadget. The halo has a mass of ≈3 × 1012 M⊙ and a virial radius of ≈270 kpc. The simulations in a Λ cold dark matter (ΛCDM) cosmology also include radiative cooling, star formation, supernova feedback and the description of non‐ideal MHD. A primordial magnetic seed field ranging from 10−10 to 10−34 G in strength agglomerates together with the gas within filaments and protohaloes. There, it is amplified within a couple of hundred million years up to equipartition with the corresponding turbulent energy. The magnetic field strength increases by turbulent small‐scale dynamo action. The turbulence is generated by the gravitational collapse and by supernova feedback. Subsequently, a series of halo mergers leads to shock waves and amplification processes magnetizing the surrounding gas within a few billion years. At first, the magnetic energy grows on small scales and then self‐organizes to larger scales. Magnetic field strengths of ≈10−6 G are reached in the centre of the halo and drop to ≈10−9 G in the intergalactic medium. Analysing the saturation levels and growth rates, the model is able to describe the process of magnetic amplification notably well and confirms the results of the simulations.

Enhancement of magnetic fields arising from galactic encounters

“…We cannot address this issue here; clearly the problem deserves specific investigation (e.g. Geng et al 2012;Beck et al 2012). We note that our results are broadly consistent with studies of cosmic ray driven dynamos (e.g.…”

“…We represent this by an increase in the parameter B inj0 , but could also consider increasing the size of the injection sites, or their number. Furthermore, we do not know the relation between energy deposited and the resulting small-scale dynamo field strength (see however Geng et al 2012;Beck et al 2012, which support the general view that something like equipartition will occcur). We stress that the point deserves clarification; however, we restrict ourselves to this simple parametrization.…”

Magnetic fields in the nearby spiral galaxy IC 342: A multi-frequency radio polarization study

“…(2) The total radio emission closely follows the dust emission in the infrared at 8 μm (Spitzer) and at 22 μm (WISE). This relation shows that the amplification of total (mostly turbulent) magnetic fields occurs in star-forming regions, probably by the turbulent dynamo (Beck et al 2012;Schleicher & Beck 2013;Pakmor et al 2014). (3) The radial distributions of the total synchrotron emission at λ6.2 cm and λ20.1 cm can be described by different exponential functions in the inner and outer disks with a break radius at about 6 , similar to the break radius in the distribution of the thermal emission and of the total neutral gas between the inner (strongly star-forming) and outer disks.…”

A new interpretation of the far-infrared – radio correlation and the expected breakdown at high redshift

“…Due to the rapid magnetic field amplification via turbulence, the small-scale dynamo was thus suggested to provide strong magnetic fields already during the formation of the first stars and galaxies (Beck et al 1996;Arshakian et al 2009;Schleicher et al 2010;de Souza & Opher 2010;Sur et al 2010;Sur et al 2012;Turk et al 2012;Schober et al 2012b;Beck et al 2012;Latif et al 2013). More recently, turbulent magnetic field amplification was suggested to be efficient even in voids (Beck et al 2013).…”