Photon dominated regions in the spiral arms of M 83 and M 51 C. Kramer, B. Mookerjea, E. Bayet, S. Garcia-Burillo, M. Gerin, F. P. Israel, J. Stutzki and J. G. A. Wouterloot A&A, 441 3 (2005) 961-973 Published online: 23 September 2005 DOI: 10.1051/0004-6361:20053358