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For refcode 1991ApJ...377..392L:
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Copyright by American Astronomical Society. Reproduced by permission
1991ApJ...377..392L COSMIC-RAY HEATING OF COOLING FLOWS: A CRITICAL ANALYSIS MICHAEL LOEWENSTEIN Joint Institute for Laboratory Astrophysics, University of Colorado and National Institute of Standards and Technology ELLEN G. ZWEIBEL Department of Astrophysical, Planetary, and Atmospheric Sciences, University of Colorado; and Joint Institute for Laboratory Astrophysics, University of Colorado and National Institute of Standards and Technology AND MITCHELL C. BEGELMAN Joint Institute for Laboratory Astrophysics, University of Colorado and National Institute of Standards and Technology; and Department of Astrophysical, Planetary, and Atmospheric Sciences, University of Colorado Received 1990 September 6; accepted 1991 February 18 ABSTRACT We present a detailed investigation of the hypothesis that a combination of magnetohydrodynamic-wave mediated cosmic-ray heating and thermal conduction might serve to balance cooling in intracluster media and substantially reduce the rate of inflow. We show that this is a particularly promising way to heat intracluster media (ICM) for a number of reasons. Because of the form of the cosmic-ray heating, a nearly static ICM with a positive temperature gradient can exist. Because of the origin of the cosmic rays-presumed to be an active nucleus in the central galaxy fueled by residual inflow-a globally stable feedback mechanism is at work, and, in contrast to the case where conduction alone operates, no unphysical fine-tuning of parameters is required. Since, while cosmic rays dominate the heating at small cluster radii, thermal conduction dominates at large radii, an (undetectably) small cosmic-ray luminosity is required. we derive and solve the appropriate system of steady state equations that include a new, self-consistent formulation for the cosmic- ray diffusivity. Models successful in producing substantial positive temperature gradients in static configurations are indeed found, but only if conduction is reduced by a factor of 10 or more. Unfortunately, these models become cosmic-ray pressure-dominated and as a result have too-flat thermal pressure profiles when compared with the observations. This negative result is confirmed by semiempirical models which solve, simultaneously, for the required cosmic-ray pressure and wave (Alfven) speed distributions. The cosmic-ray pressure gradient can be reduced to acceptable levels only for central values of the Alfven speed considerably in excess of the local thermal sound speed. In such a case the magnetic pressure can no longer be justifiably neglected. in the radial force equation, since the required reduction in conduction is presumably a result of tangled magnetic fields; this pressure would again lead to a thermal gas distribution flatter than observed. The effect of cosmic rays on the thermal stability of the ICM is also investigated, as is the role of cosmic rays in heating emission-line filaments. We find that cosmic-ray heating is unlikely either to stabilize positive- density perturbations against condensation or to contribute appreciably to the powering of the optical filaments. Subject headings: cosmic rays: general - galaxies: intergalactic medium - hydromagnetics
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