Cluster Magnetic Fields from Large-Scale Structure and Galaxy Cluster Shocks

Abstract

The origin of the micro-Gauss magnetic fields in galaxy clusters is one of the outstanding problem of modern cosmology. We have performed three-dimensional particle-in-cell simulations of the nonrelativistic Weibel instability in an electron-proton plasma, in conditions typical of cosmological shocks. These simulations indicate that cluster fields could have been produced by shocks propagating through the intergalactic medium during the formation of large-scale structure or by shocks within the cluster. The strengths of the shock-generated fields range from tens of nano-Gauss in the intercluster medium to a few micro-Gauss inside galaxy clusters. 95.30.Qd, 52.65.Rr, 52.35.Qz, 52.52.Tc The origin of the micro-Gauss magnetic fields observed in galaxy clusters [1,2,3] poses one of the most intriguing problems in modern cosmology. The most common explanation invokes the amplification of seed or primordial fields by hydrodynamic turbulence that have been excited during the processes of large-scale-structure (LSS) formation. Although there are viable astrophysical mechanisms that can generate seed fields with B ∼ 10 −16 Gauss or weaker [4,5,6,7], recent cosmological simulations [8] show that structure formation can amplify the field by no more than a factor of 10 3 . Hence, in order to explain the observed intergalactic field, one needs seed fields as strong as B ∼ 10 −9 Gauss. Scenarios with galactic winds and quasar-driven outflows [9, 10] can provide fields of such strength, but they are rather localized. It is thus not clear whether they can explain entirely the origin of the intergalactic fields in galaxy clusters.Here we show that magnetic fields can be produced by collisionless shocks in galaxy clusters and in the intercluster medium (ICM) during LSS formation. Cosmological N -body and hydrodynamic simulations of LSS formation [11,12] have shown that shocks with Mach numbers up to M ∼ 100 are ubiquitous on scales of few to few tens of megaparsecs. Theoretical analysis of non-magnetized collisionless shocks indicates that they can generate subequipartition fields [13,14,15]. We verify this prediction with state-of-the-art numerical simulations. We present here three-dimensional (3D) particle-in-cell (PIC) simulations of the nonrelativistic (with v = 0.1c) Weibel instability [16,17] in an electron-proton (with m p /m e = 100) plasma, thus guaranteeing a clear separation of the relevant time scales. These simulations are computationally expensive and represent a significant advance over previous studies, which simulated relativistic shocks (v ∼ c) in an electron-positron or low-mass-ratio electron-ion plasmas (m i /m e ≤ 16) [18,19,20]. Note that a recently discussed possibility that cluster shocks may produce the magnetic fields seen in galaxy clusters [21] was based on the assumption that the results of relativistic simulations will also apply in the nonrelativistic regime. Our nonrel-ativistic simulations fully confirm theoretical predictions and indicate that LSS shocks can produce magnetic fields of streng...

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