Cosmic ray feedback in the FIRE simulations: constraining cosmic ray propagation with GeV γ-ray emission

We present the implementation and the first results of cosmic ray (CR) feedback in the Feedback In Realistic Environments (FIRE) simulations. We investigate CR feedback in non-cosmological simulations of dwarf, sub-L star starburst, and L star galaxies with different propagation models, including advection, isotropic, and anisotropic diffusion, and streaming along field lines with different transport coefficients. We simulate CR diffusion and streaming simultaneously in galaxies with high resolution, using a two-moment method. We forward model and compare to observations of gamma-ray emission from nearby and starburst galaxies. We reproduce the gamma-ray observations of dwarf and L star galaxies with constant isotropic diffusion coefficient kappa similar to 3 x 10(29) cm(2) s(-1). Advection-only and streaming-only models produce order of magnitude too large gamma-ray luminosities ill dwarf and L star galaxies. We show that in models that match the gamma-ray observations, most CRs escape low-gas-density galaxies (e.g. dwarfs) before significant collisional losses, while starburst galaxies are CR proton calorimeters. While adiabatic losses can be significant, they occur only after CRs escape galaxies, so they are only of secondary importance for gamma-ray emissivities. Models where CRs are 'trapped' in the star-forming disc have lower star formation efficiency, but these models are ruled out by gamma-ray observations. For models with constant w that match the gamma-ray observations, CRs form extended haloes with scale heights of several kpc to several tens of kpc.