Abstract. We use cosmological magneto-hydrodynamic simulations to study the evolution of magnetic fields in galaxy clusters in two different cosmological models, a standard-CDM and a Λ-CDM model. We show that the magnetic field strength profiles closely follow the cluster density profiles outside a core region of radius ∼200 h −1 kpc. The magnetic field has a correlation length of order 50 h −1 kpc and reverses on scales of ∼100 h −1 kpc along typical lines-of-sight. The power spectrum of the magnetic field can well be approximated by a steep power law with an exponent of ∼−2.7. The mean magnetic field in the cluster cores grows roughly exponentially with decreasing redshift, B ∼ 10 −2.5z µG. Merger events have a pronounced effect on magnetic field evolution, which is strongly reflected in measurable quantities like the Faraday rotation. The field evolution in the two different cosmologies proceeds virtually identically. All our cluster models very well reproduce observed Faraday rotation measurements when starting with nG seed fields.

We performed cosmological, magnetohydrodynamical simulations to follow the evolution of magnetic fields in galaxy clusters, exploring the possibility that the origin of the magnetic seed fields is galactic outflows during the starburst phase of galactic evolution. To do this, we coupled a semi‐analytical model for magnetized galactic winds as suggested by Bertone, Vogt & Enßlin to our cosmological simulation. We find that the strength and structure of magnetic fields observed in galaxy clusters are well reproduced for a wide range of model parameters for the magnetized, galactic winds and do only weakly depend on the exact magnetic structure within the assumed galactic outflows. Although the evolution of a primordial magnetic seed field shows no significant differences to that of galaxy cluster fields from previous studies, we find that the magnetic field pollution in the diffuse medium within filaments is below the level predicted by scenarios with pure primordial magnetic seed field. We therefore conclude that magnetized galactic outflows and their subsequent evolution within the intracluster medium can fully account for the observed magnetic fields in galaxy clusters. Our findings also suggest that measuring cosmological magnetic fields in low‐density environments such as filaments is much more useful than observing cluster magnetic fields to infer their possible origin.

We present simulations of the magnetized interstellar medium (ISM) in models of massive star forming (40 M ⊙ yr −1 ) disk galaxies with high gas surface densities (Σ gas ∼ 100 M ⊙ pc −2 ) similar to observed star forming high-redshift disks. We assume that type II supernovae deposit 10 per cent of their energy into the ISM as cosmic rays and neglect the additional deposition of thermal energy or momentum. With a typical Galactic diffusion coefficient for CRs (3 · 10 28 cm 2 s −1 ) we demonstrate that this process alone can trigger the local formation of a strong low density galactic wind maintaining vertically open field lines. Driven by the additional pressure gradient of the relativistic fluid the wind speed can exceed 10 3 km s −1 , much higher than the escape velocity of the galaxy. The global mass loading, i.e. the ratio of the gas mass leaving the galactic disk in a wind to the star formation rate becomes of order unity once the system has settled into an equilibrium. We conclude that relativistic particles accelerated in supernova remnants alone provide a natural and efficient mechanism to trigger winds similar to observed mass-loaded galactic winds in high-redshift galaxies. These winds also help explaining the low efficiencies for the conversion of gas into stars in galaxies as well as the early enrichment of the intergalactic medium with metals. This mechanism can be at least of similar importance than the traditionally considered momentum feedback from massive stars and thermal and kinetic feedback from supernova explosions.

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Particle acceleration in collisionless magnetic reconnection is studied in the relativistic regime of an electron-positron plasma. For the first time, the highly dynamic late-time evolution of reconnection is simulated in two dimensions (2D) and the finite size of the acceleration region is resolved in 3D applying a fully electromagnetic relativistic particle-in-cell (PIC) code. The late-time evolution is extremely important with respect to particle acceleration, because thin current sheets show a highly dynamic late-time phase with instabilities evolving in the Alfvén velocity vA0 regime. Consequently, since c∼vA0 is valid as a peculiarity of pair plasmas, v×B-contributions become dominant in the accelerating electric field. Most remarkable: Though acceleration regions are highly variable at late times, the power-law shape of the particle energy distribution is smoothed compared to quasi-static reconnection configurations at early times [S. Zenitani and M. Hoshino, Astrophys. J. 562, L63 (2001)]. Spectral power indices of s∼−3 for the complete simulation box, s∼−1 within the X-zone, are preserved at late times and appear as a characteristic of pair plasma reconnection of thin current sheets! The spectral high-energy cut-off depends on the sheet width at late times and is most efficiently tuned by the ratio c/vA0. In 3D, sheet instabilities limit the acceleration potential of a single X-zone, but current driven instabilities like the relativistic drift kink mode can also significantly contribute to particle acceleration. Via the analysis of particle trajectories, the consequences of a finite 3D acceleration zone are resolved and efficient acceleration mechanisms in the context of dynamic X-points are identified.