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Register a new userCosmic Ray Protons and Magnetic Fields in Clusters of Galaxies and their Cosmological Consequences
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T.A. Ensslin
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The masses of clusters of galaxies estimated by gravitational lensing exceed in many cases the mass estimates based on hydrostatic equilibrium. This may suggest the existence of nonthermal pressure. We ask if radio galaxies can heat and support the cluster gas with injected cosmic ray protons and magnetic field densities, which are permitted by Faraday rotation and gamma ray observations of clusters of galaxies. We conclude that they are powerful enough to do this within a cluster radius of roughly 1 Mpc. If present, nonthermal pressures could lead to a revised estimate of the ratio of baryonic mass to total mass, and the apparent baryonic overdensity in clusters would disappear. In consequence, $Omega_{rm cold}$, the clumping part of the cosmological density $Omega_{o}$, would be larger than $0.4,h_{50}^{-1/2}$.
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Numerical simulations of the propagation of charged particles through magnetic fields solving the equation of motion often leads to the usage of an interpolation in case of discretely defined magnetic fields, typically given on a homogeneous grid structure. However, the interpolation method influences the magnetic field properties on the scales of the grid spacing and the choice of interpolation routine can therefore change the result. At the same time, it provides an impact, i.e. error, on the spatial particle distribution. We compare three different interpolation routines -- trilinear, tricubic and nearest neighbor interpolation -- in the case of turbulent magnetic fields and show that there is no benefit in using trilinear interpolation. We show that in comparison, the nearest neighbor interpolation provides the best performance, i.e. requires least CPU time and results in the smallest error. In addition, we optimize the performance of an algorithm that generates a continuous grid-less turbulent magnetic field by more than an order of magnitude. This continuous method becomes practicable for the simulation of large particle numbers and its accuracy is only limited by the used number of wave-modes. We show that by using more than 100 wave-modes the diffusive behavior of the spatial particle distribution in form of the diffusion coefficient is determined with an error less than a few percentage.
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An important area of study of cosmic magnetic fields is on the largest scales, those of clusters of galaxies. In the last decade it has become clear that the intra-cluster medium (ICM) in clusters of galaxies is magnetized and that magnetic fields play a critical role in the cluster formation and evolution. The observational evidence for the existence of cluster magnetic fields is obtained by the diffuse cluster-wide synchrotron radio emission and from rotation measure (RM) studies of extragalactic radio sources located within or behind the clusters. A significant breakthrough in the knowledge of the cluster magnetic fields will be reached through the SKA, owing to its capabilities, in particular the deep sensitivity and the polarization purity.
We use numerical simulations of large scale structure formation to explore the cosmological properties of Gamma-Ray Burst (GRB) host galaxies. Among the different sub-populations found in the simulations, we identify the host galaxies as the most efficient star-forming objects, i.e. galaxies with high specific star formation rates. We find that the host candidates are low-mass, young galaxies with low to moderate star formation rate. These properties are consistent with those observed in GRB hosts, most of which are sub-luminous, blue galaxies. Assuming that host candidates are galaxies with high star formation rates would have given conclusions inconsistent with the observations. The specific star formation rate, given a galaxy mass, is shown to increase as the redshift increases. The low mass of the putative hosts makes them difficult to detect with present day telescopes and the probability density function of the specific star formation rate is predicted to change depending on whether or not these galaxies are observed.
We examine the cosmic-ray protons (CRp) accelerated at collisionless shocks in galaxy clusters using cosmological structure formation simulations. We find that in the intracluster medium (ICM) within the virial radius of simulated clusters, only $sim7$% of shock kinetic energy flux is dissipated by the shocks that are expected to accelerate CRp, that is, supercritical, quasi-parallel ($Q_parallel$) shocks with sonic Mach number $M_sge2.25$. The rest is dissipated at subcritical shocks and quasi-perpendicular shocks, both of which may not accelerate CRp. Adopting the diffusive shock acceleration (DSA) model recently presented in Ryu et al. (2019), we quantify the DSA of CRp in simulated clusters. The average fraction of the shock kinetic energy transferred to CRp via DSA is assessed at $sim(1-2)times10^{-4}$. We also examine the energization of CRp through reacceleration using a model based on the test-particle solution. Assuming that the ICM plasma passes through shocks three times on average through the history of the universe and that CRp are reaccelerated only at supercritical $Q_parallel$-shocks, the CRp spectrum flattens by $sim0.05-0.1$ in slope and the total amount of CRp energy increases by $sim40-80$% from reacceleration. We then estimate diffuse $gamma$-ray and neutrino emissions, resulting from inelastic collisions between CRp and thermal protons. The predicted $gamma$-ray emissions from simulated clusters lie mostly below the upper limits set by Fermi-LAT for observed clusters. The neutrino fluxes towards nearby clusters would be $lesssim10^{-4}$ of the IceCube flux at $E_{ u}=1$ PeV and $lesssim10^{-6}$ of the atmospheric neutrino flux in the energy range of $E_{ u}leq1$ TeV.
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