Modeling Cosmic-ray Electron Spectra and Synchrotron Emission in the Multiphase Interstellar Medium

We model the transport and spectral evolution of 1-100 GeV cosmic-ray electrons (CREs) in TIGRESS magnetohydrodynamic simulations of the magnetized, multiphase interstellar medium. We postprocess a kpc-sized galactic disk patch representative of the solar neighborhood using a two-moment method for cosmic ray (CR) transport that includes advection, streaming, and diffusion. The diffusion coefficient is set by balancing wave growth via the CR streaming instability against wave damping (nonlinear Landau and ion-neutral collisions), depending on local gas and CR properties. Implemented energy loss mechanisms include synchrotron, inverse Compton, ionization, and bremsstrahlung. We evaluate CRE losses by different mechanisms as a function of energy and distance from the midplane, and compare loss timescales to transport and diffusion timescales. This comparison shows that CRE spectral steepening above p = 1 GeV c−1 is due to a combination of energy-dependent transport and losses. Our evolved CRE spectra are consistent with direct observations in the solar neighborhood, with a spectral index that steepens from an injected value of −2.3 to an energy-dependent value between −2.7 and −3.3. We also show that the steepening is independent of the injection spectrum. Finally, we present potential applications of our models, including to the production of synthetic synchrotron emission. Our simulations demonstrate that the CRE spectral slope can be accurately recovered from pairs of radio observations in the range 1.5-45 GHz.