Session H3: Magnetoplasmas in Astrophysical Jets, Lobes, and in the Laboratory

Global Structures of Radio Galaxies -- Theory and Simulations Meet Observations

X-ray and radio observations of galaxy clusters have revealed a wealth of structure in their hot halos associated with extragalactic radio sources. Structures in the form of large scale cavities and weak shocks provide a reliable gauge of the mechanical output of extragalactic radio jets launched by AGNs. The energies involved range between 1E57 to 1E62 ergs. Furthermore, the morphology and properties of cavities have given strong constraints on the nature of AGN outflows, especially on large scales. We will present 3-D magnetohydrodynamic (MHD) simulations to study these large scale structures of radio galaxies, emphasizing the roles of magnetic fields and kinetic energy flow. The important effects of background environment on the radio galaxies will be discussed. In addition, we will present self-consistent cosmological MHD simulations of cluster formation with AGN feedback, emphasizing the important role of magnetic fields in carrying the AGN energy and in the cavity formation. Such simulations are compared with cluster radio halo and relic observations, as well as extensive Faraday rotation measurements. These results are sheding light on the origin and energetics of the cluster-wide magnetic fields. We demonstrate that the intracluster medium turbulence can be excited and sustained by the frequent mergers during the cluster formation. This turbulence then excites a small-scale dynamo process that transports, spreads, and amplifies the fields originated from the radio jet/lobe system. This process could be the primary process of populating the whole cluster with magnetic fields at observed levels.   

Exploring how astrophysical jets work using laboratory plasma jets

Astrophysical jets occur in numerous contexts where there is accretion (e.g., stellar formation, black holes) and are presumed to be driven by magnetohydrodynamic (MHD) forces. This talk describes a laboratory plasma experiment that simulates the essential features of astrophysical jets. The geometry is arranged so the laboratory jets are unaffected by walls and the experimental time scale is such that frozen-in magnetic flux, the condition for ideal MHD, is reasonably approximated. The lab jets evolve through a sequence of reproducible stages consisting of formation, collimation, kink instability, and at sufficiently high electric current, detachment. Diagnostics include imaging at $>$ 1 million frames per second, magnetic probing, spectroscopy, and laser interferometry. The collimated nature of both these jets and of arched plasma-filled flux tubes in a related solar corona loop simulation experiment suggest that collimation is a ubiquitous property of magnetic flux tubes conducting axial electric currents. This realization has motivated a collimation mechanism model whereby the accumulation of convected, frozen-in toroidal magnetic flux near the jet tip increases the toroidal magnetic flux density near the tip. Since magnetic flux density is magnetic field strength, this flux pile-up corresponds to an increase of the toroidal field near the tip. Increase of toroidal field increases the MHD pinch force thereby collimating the jet. The model additionally shows that plasma-filled coronal loops can be considered as resulting from two counter-propagating jets colliding head-on; color-coded images of two colliding lab jets confirm this postulate. The experiments have also motivated development of a dusty-plasma dynamo mechanism suitable for driving actual astrophysical jets. This mechanism involves dust grains having a charge to mass ratio so small that their cyclotron frequency becomes comparable to the Kepler frequency. The resulting collisionless orbits spiral across magnetic field lines towards the central object and the accumulation of charged dust grains creates a radial electromotive force appropriate for driving an astrophysical jet. These spiral orbits are not described by MHD but instead result from conservation of canonical angular momentum in combined gravitational and magnetic fields.   

Extended Lobes as Sources of High Energy Particles and Radiation

Extended lobes in radio galaxies and quasars are one of the largest structures in the Universe, in many cases reaching enormous sizes of a few billion parsecs ($\sim 10^{25}$\,cm). These are formed by relativistic jets emanating from the closest vicinities of supermassive black holes in the centers of active galaxies, and interacting with the ambient intergalactic medium. The physical conditions in extended lobes are hardly known, although it is established that the lobes are filled with ionized, highly magnetized, and rarefied plasma extracted predominantly from the surrounding medium of supermassive black holes. Presence of magnetic turbulence and extended shock waves in the lobes ensures efficient acceleration of plasma particles up to the very high, ultrarelativistic energies. Such energies are not, and will be not accessible in our laboratories even in a near future. In this talk I will summarize present understanding of the lobes' structure and evolution, and I will also review the recently discussed ideas and models regarding particle acceleration and generation of high-energy radiation therein. Finally, I will also discuss the current status and prospects of high-energy observations of these extreme structures, focusing especially on the X-ray and $\gamma$-ray domains.