The excitation of the interstellar gas. is discussed in the light of recent evidence from γ-ray, molecular, and 21-cm line observations. Previous studies of the excitation of the interstellar gas have not taken into account the substantial density contrast that exists between spiral arms and interarm regions. We examine the role played by the galactic distribution of three sources of excitation (supernovae, OB stars, and ultraviolet stars) in determining the physical state of the interstellar gas in arm and interarm regions.
Cowan, J.J., Kafatos, M., Rose, W.K. (1975) Sources of Excitation of the Interstellar Gas and Galactic Structure, Astrophysical Journal, 195: 47-51. doi: 10.1086/153302