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Published online by Cambridge University Press: 12 April 2016
We present aperture synthesis maps of the CO J=2-1 emission in the central region of the spiral galaxy IC 342. The 4” resolution maps reveal emission that is a factor of two brighter than the CO (1-0) emission mapped at the same resolution. Since the CO (2-1) emission is likely to be optically thick, the high ratio is probably due to the fact that the two transitions sample different cloud layers in externally heated clouds. The high signal to noise of the maps indicates that CO (2-1) will be a powerful tool in the study of gas in galaxies.