Francesco Miniati

Properties of cosmic shock waves in large scale structure formation

We have examined the properties of shock waves in simulations of large-scale structure formation. Two cosmological scenarios have been considered: a standard cold dark matter model with ΩM = 1 (SCDM), and a cold dark matter model with cosmological constant and ΩM + ΩΛ = 1 (ΛCDM) having ΩΛ = 0.55. Large-scale shocks result from accretion onto sheets, filaments, and knots of mass distribution on a scale of the order of ~5 h-1 Mpc in both scenarios. Energetic motions, partly residuals of past accretion processes and partly caused by current asymmetric inflow along filaments, end up generating additional shocks. These extend on a scale of the order of ~1 h-1 Mpc and envelop and penetrate deep inside the clusters. Collisions between substructures inside clusters also form merger shocks. Consequently, the topology of the shocks is very complex and highly connected. During cosmic evolution the comoving shock surface density decreases, reflecting the ongoing structure merger process in both scenarios. Accretion shocks have very high Mach numbers, typically between 10 and a few ×103, when photoheating of the preshock gas is not included. The characteristic shock velocity is of the order of vsh(z) = H(z)λnl(z), where λnl(z) is the wavelength scale of the nonlinear perturbation at the given epoch. However, the Mach number for merger and flow shocks (which occur within clusters) is usually smaller, in the range of ~3-10, corresponding to the fact that the intracluster gas is hot (i.e., already shock heated). Statistical fits of shock velocities around clusters as a function of cluster temperature give power-law functions in accord with those predicted by one-dimensional solutions. On the other hand, a very different result is obtained for the shock radius, reflecting extremely complex shock structures surrounding clusters of galaxies in three-dimensional simulations. The amount of inflowing kinetic energy across the shocks around clusters, which represents the power available for cosmic-ray acceleration, is comparable to the cluster X-ray luminosity emitted from a central region of radius 0.5 h-1 Mpc. Considering their large size and long lifetimes, those shocks are potentially interesting sites for cosmic-ray acceleration, if modest magnetic fields exist within them.

Cosmic-Ray Electrons in Groups and Clusters of Galaxies: Primary and Secondary Populations from a Numerical Cosmological Simulation

We investigate the generation and distribution of high-energy electrons in the cosmic structure environment and their observational consequences by carrying out the first cosmological simulation that includes directly cosmic-ray (CR) particles. Starting from cosmological initial conditions, in addition to the gas and dark matter related quantities, we follow the evolution of CR electrons (primary and secondary) and CR ions along with a passive magnetic field. CR ions and primary electrons are injected in accordance with the thermal leakage model and accelerated in the test-particle limit of diffusive shock acceleration at shocks associated with large-scale structure formation. Secondary electrons are continuously generated through p-p inelastic collisions of the CR ions with the thermal nuclei of the intergalactic medium. The evolution of the CR electrons accounts for spatial transport, adiabatic expansion/compression, and losses due to Coulomb collisions, bremsstrahlung, synchrotron and inverse-Compton emission. The magnetic field is seeded at shocks according to the Biermann battery model, and thereafter amplified by shear flow and gas compression. We compute the emission due to the inverse-Compton scattering of the simulated primary and secondary electrons off cosmic microwave background photons and compare it with the published values of the detected radiation excesses in the hard X-ray and extreme-ultraviolet wavebands. We find that the few instances of detection of hard X-ray radiation excess could be explained in the framework of IC emission from primary electrons in clusters characterized by high accretion/merger activity. On the other hand, with the only exception of measured flux from the Coma Cluster by Bowyer, Berghoefer & Korpela, both primary and secondary CR electrons associated with the cosmic structure formation account at most for a small fraction of the radiation excess detected in the extreme-ultraviolet waveband. Next, we calculate the synchrotron emission after normalizing the magnetic field strength so that for a Coma-like cluster the volume-averaged B21/2 3 ?G. Our results indicate that the synchrotron emission from the secondary CR electrons reproduces several general properties observed in radio halos. These include the recently found P1.4 GHz versus TX relationship, the morphology and polarization of the emitting region, and, to some extent, even the spectral index. In addition, radio synchrotron emission from primary electrons turns out to be large enough to power extended regions of radio emission, resembling radio relics observed at the outskirts of clusters. Once again we find a striking resemblance between the general properties of morphology, polarization, and spectral index of our synthetic maps and those of reported in the literature.

A divergence-free upwind code for multidimensional magnetohydrodynamic flows

A description is given for preserving ∇ = 0 in a magnetohydrodynamic (MHD) code that employs the upwind, total variation diminishing (TVD) scheme and Strang type operator splitting for multidimensionality. The method is based on the staggered mesh technique to constrain the transport of magnetic field: the magnetic field components are defined at grid interfaces with their advective fluxes on grid edges, while other quantities are defined at grid centers. The magnetic field at grid centers for the upwind step is calculated by interpolating the values from grid interfaces. The advective fluxes on grid edges for the magnetic field evolution are calculated from the upwind fluxes at grid interfaces. Then the magnetic field can be maintained with ∇ = 0 exactly, if this is so initially, while the upwind scheme is used for the update of fluid quantities. The correctness of the code is demonstrated through tests comparing numerical solutions either with analytic solutions or with numerical solutions from a code using an explicit divergence-cleaning method. Also, the robustness is shown through tests involving realistic astrophysical problems.

Cosmic-Ray Protons Accelerated at Cosmological Shocks and Their Impact on Groups and Clusters of Galaxies

We investigate the production of cosmic ray (CR) protons at cosmological shocks by performing, for the first time, numerical simulations of large scale structure formation that include directly the acceleration, transport and energy losses of the high energy particles. CRs are injected at shocks according to the thermal leakage model and, thereafter, accelerated to a power-law distribution as indicated by the test particle limit of the diffusive shock acceleration theory. The evolution of the CR protons accounts for losses due to adiabatic expansion/compression, Coulomb collisions and inelastic p-p scattering. Our results suggest that CR protons produced at shocks formed in association with the process of large scale structure formation could amount to a substantial fraction of the total pressure in the intra-cluster medium. Their presence should be easily revealed by GLAST through detection of gamma-ray flux from the decay of neutral pions produced in inelastic p-p collisions of such CR protons with nuclei of the intra-cluster gas. This measurement will allow a direct determination of the CR pressure contribution in the intra-cluster medium. We also find that the spatial distribution of CR is typically more irregular than that of the thermal gas because it is more influenced by the underlying distribution of shocks. This feature is reflected in the appearance of our gamma-ray synthetic images. Finally, the average CR pressure distribution appears statistically slightly more extended than the thermal pressure.

COSMOCR: A numerical code for cosmic ray studies in computational cosmology

We present COSMOCR, a numerical code for the investigation of cosmic ray related studies in computational cosmology. The code follows the diffusive shock acceleration, the mechanical and radiative energy losses and the spatial transport of the supra-thermal particles in cosmic environment. Primary cosmic ray electrons and ions are injected at shocks according to the thermal leakage prescription. Secondary electrons are continuously injected as a results of p-p inelastic collisions of primary cosmic ray ions and thermal background nuclei. The code consists of a conservative, finite volume method with a power-law sub-grid model in momentum space. Two slightly different schemes are implemented depending on the stiffness of the cooling terms. Comparisons of numerical results with analytical solution for a number of tests of direct interest show remarkable performance of the present code.

THE MATRYOSHKA RUN. II. TIME-DEPENDENT TURBULENCE STATISTICS, STOCHASTIC PARTICLE ACCELERATION, AND MICROPHYSICS IMPACT IN A MASSIVE GALAXY CLUSTER

We use the Matryoshka run to study the time dependent statistics of structure-formation driven turbulence in the intracluster medium of a 10

Enhanced cloud disruption by magnetic field interaction

^{15}M_odot

Hydrodynamics of Cloud Collisions in Two Dimensions: The Fate of Clouds in a Multiphase Medium

galaxy cluster. We investigate the turbulent cascade in the inner Mpc for both compressional and incompressible velocity components. The flow maintains approximate conditions of fully developed turbulence, with departures thereof settling in about an eddy-turnover-time. Turbulent velocity dispersion remains above

Magnetohydrodynamics of Cloud Collisions in a Multiphase Interstellar Medium

700

Hydrodynamics of Cloud Collisions in 2D: The Fate of Clouds in a Multi-phase Medium

km s