Oct 07, 2013
Cosmic Ray Sampling of a Clumpy Interstellar Medium
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. I will discuss the use of 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. We find that the cosmic rays sample approximately the mean density of the ISM over much of the parameter space considered; however, we are able to achieve sampling of densities a factor of a few higher than the mean density by injecting the cosmic rays close to very dense molecular clouds in the presence of a very tangled magnetic field. I will discuss applications of our simulation results, including evaluating the cosmic ray calorimeter model for starburst galaxies.