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Whistler-regulated Magnetohydrodynamics: Transport Equations for Electron Thermal Conduction in the High-β Intracluster Medium of Galaxy Clusters
Transport equations for electron thermal energy in the high
intracluster medium (ICM) are developed that include scattering from both
classical collisions and self-generated whistler waves. The calculation employs
an expansion of the kinetic electron equation along the ambient magnetic field
in the limit of strong scattering and assumes whistler waves with low phase
speeds dominate the turbulent spectrum,
with the electron thermal speed and the ratio of
electron thermal to magnetic pressure. We find: (1) temperature-gradient-driven
whistlers dominate classical scattering when , with the
classical electron mean-free-path and the electron temperature scale
length, and (2) in the whistler dominated regime the electron thermal flux is
controlled by both advection at and a comparable diffusive term. The
findings suggest whistlers limit electron heat flux over large regions of the
ICM, including locations unstable to isobaric condensation. Consequences
include: (1) the Field length decreases, extending the domain of thermal
instability to smaller length-scales, (2) the heat flux temperature dependence
changes from to , (3) the magneto-thermal
and heat-flux driven buoyancy instabilities are impaired or completely
inhibited, and (4) sound waves in the ICM propagate greater distances, as
inferred from observations. This description of thermal transport can be used
in macroscale ICM models
Theoretical prediction on mechanical and thermal properties of a promising thermal barrier material: Y4Al2O9
Effects of pressure on the structural, mechanical, anisotropic, and electronic properties of via density functional theory
SEARCH FOR COSMIC-RAY-INDUCED GAMMA-RAY EMISSION IN GALAXY CLUSTERS
Current theories predict relativistic hadronic particle populations in clusters of galaxies in addition to the already observed relativistic leptons. In these scenarios hadronic interactions give rise to neutral pions which decay into gamma rays that are potentially observable with the Large Area Telescope (LAT) on board the Fermi space telescope. We present a joint likelihood analysis searching for spatially extended gamma-ray emission at the locations of 50 galaxy clusters in four years of Fermi-LAT data under the assumption of the universal cosmic-ray (CR) model proposed by Pinzke & Pfrommer. We find an excess at a significance of 2.7 sigma, which upon closer inspection, however, is correlated to individual excess emission toward three galaxy clusters: A400, A1367, and A3112. We discuss these cases in detail and conservatively attribute the emission to unmodeled background systems (for example, radio galaxies within the clusters). Through the combined analysis of 50 clusters, we exclude hadronic injection efficiencies in simple hadronic models above 21% and establish limits on the CR to thermal pressure ratio within the virial radius, R-200, to be below 1.25%-1.4% depending on the morphological classification. In addition, we derive new limits on the gamma-ray flux from individual clusters in our sample