The PAH 7.7 μm/850 μm ratio as new diagnostics for high extinction in ULIRGs -increasing evidence for a hidden quasar in Arp 220M. Haas, U. Klaas, S. A. H. Müller, R. Chini and I. CoulsonA&A, 367 3 (2001) L9-L13DOI: https://doi.org/10.1051/0004-6361:20010065
