Distribution of gas, magnetic fields, and cosmic rays in the galaxy

We present a tentative picture of the large-scale distribution of gas, magnetic fields, and cosmic rays in the galactic disk, derived from high-energy γ-ray and nonthermal radio surveys supplemented by radio observations on M31. The interpretation of the observational material suggests that, on a large scale, the cosmic-ray density and the square of the magnetic field strength are both proportional to the gas density. The distribution of the gas along a spiral pattern is derived from the radio and γ-ray data; the results are consistent with an important amount of molecular hydrogen in the inner Galaxy. Given the local cosmic-ray intensity, the model yields local values for the magnetic field strength, the gas density, and the velocity dispersion which are in agreement with independent evaluations. Assuming that cosmic-ray injection is related to the gas density, we discuss further implications regarding cosmic-ray creation and propagation