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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-$Lstar$ starburst, and $Lstar$ 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 $Lstar$ galaxies with constant isotropic diffusion coefficient $kappa sim 3times 10^{29},{rm cm^{2},s^{-1}}$. Advection-only and streaming-only models produce order-of-magnitude too large $gamma$-ray luminosities in dwarf and $Lstar$ 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 disk have lower star formation efficiency, but these models are ruled out by $gamma$-ray observations. For models with constant $kappa$ that match the $gamma$-ray observations, CRs form extended halos with scale heights of several kpc to several tens of kpc.
We review numerical methods for simulations of cosmic ray (CR) propagation on galactic and larger scales. We present the development of algorithms designed for phenomenological and self-consistent models of CR propagation in kinetic description based
The secondary-to-primary B/C ratio is widely used to study Galactic cosmic-ray propagation processes. The 2H/4He and 3He/4He ratios probe a different Z/A regime, therefore testing the `universality of propagation. We revisit the constraints on diffus
The Tibet ASgamma experiment just reported their measurement of sub-PeV diffuse gamma ray emission from the Galactic disk, with the highest energy up to 957 TeV. These gamma-rays are most likely the hadronic origin by cosmic ray interaction with inte
Cosmic-rays constitute the main ionising and heating agent in dense, starless, molecular cloud cores. We reexamine the physical quantities necessary to determine the cosmic-ray ionisation rate (especially the cosmic ray spectrum at E < 1 GeV and the
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, t