congruents (COsmic ray, Neutrino, Gamma-ray, and Radio Non-Thermal Spectra) - I. A predictive model for galactic non-thermal emission

The total luminosity and spectral shape of the non-thermal emission produced by cosmic rays depends on their interstellar environment, a dependence that gives rise to correlations between galaxies' bulk properties - star-formation rate, stellar mass, and others - and their non-thermal spectra. Understanding the physical mechanisms of cosmic ray transport, loss, and emission is key to understanding these correlations. Here, in the first paper of the series, we present a new method to compute the non-thermal spectra of star-forming galaxies, and describe an open-source software package - congruents(COsmic ray, Neutrino, Gamma-ray, and Radio Non-Thermal Spectra) - that implements it. As a crucial innovation, our method requires as input only a galaxy's effective radius, star-formation rate, stellar mass, and redshift, all quantities that are readily available for large samples of galaxies and do not require expensive, spatially resolved gas measurements. From these inputs we derive individual, galaxy-by-galaxy models for the background gas and radiation field through which cosmic rays propagate, from which we compute steady-state cosmic ray spectra for hadronic and leptonic particles in both the galactic disc and halo by solving the full kinetic equation. We invoke modern models for cosmic ray transport and include all significant emission and loss mechanisms. In this paper, we describe the model and validate it against non-thermal emission measured in nearby star-forming galaxies that span four orders of magnitude in star-formation rate.