Do Cosmic Rays Sample the Mean ISM Density of Starburst Galaxies?

Type Conference Paper
Names Erin Boettcher, E. G. Zweibel, T. Yoast-Hull, J. S. Gallagher
Proceedings Title AAS Meeting #223, #252.08
Conference Name American Astronomical Society
Volume 223
Date January 1, 2014
Library Catalog
Abstract In studies of interacting cosmic rays and the interstellar medium (ISM) in starburst galaxies, it is often assumed that cosmic rays sample the mean density of the ISM. However, given the very high galactic wind speeds and the very small filling factors of fragmented molecular clouds, this is far from a foregone conclusion in starburst environments. Here, we use Monte Carlo simulations to assess the assumption that cosmic rays sample the mean density of a two-phase ISM consisting of molecular clouds embedded in a hot, low density medium. We simulate cosmic ray propagation in a tangled magnetic field with vertical advection and a variety of injection scenarios in a medium with properties similar to those of the prototypical starburst galaxy M82. The ratio of the sampled density to the mean density is calculated by comparing the gamma-ray emissivity from pion production in molecular clouds implied by our simulations and by cosmic ray sampling of the mean density. This ratio remains close to unity over a wide range of conditions on the number of molecular clouds, the galactic wind speed, the magnetic field geometry, and the cosmic ray injection mechanism. However, this ratio becomes elevated by a factor of a few when the cosmic rays are injected close to a small number of dense molecular clouds in the presence of a very tangled magnetic field. Using the results of our simulations, we evaluate the cosmic ray calorimeter model for starburst galaxies, and we argue that our simulated starburst region is at best a partial proton calorimeter. We acknowledge the support of NSF AST-0907837 and NSF PHY-0821899.
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