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L.P. Crosthwaite, J.L. Turner (UCLA), P.T.P. Ho (SAO/CFA), R.N. Martin (ASIAA)
We present NRAO 12 Meter Telescope "OTF" maps of the low J rotational lines of CO in M83 and NGC 6946. When combined with VLA HI 21 cm line observations we obtain large scale maps of the total neutral gas in these galaxies. From the ratio of CO(2-1) to CO(1-0) line emission, Rco, we find that the CO in these galaxies has unusual properties by Galactic standards. High mean Rco values and, in particular, Rco values in interarm regions suggest large scale optically thin CO is present. But we also consider other explanations for the unusually high Rco values. This work was supported in part by NSF grant AST00-71276 to J.L.T.