  We present spatial variations of the CO J=2-1/1-0 line ratio in M83 using
Total Power array data from ALMA. While the intensities of these two lines
correlate tightly, the ratio varies over the disk, with a disk average ratio of
0.69, and shows the galactic center and a two-arm spiral pattern. It is high
(>0.7) in regions of high molecular gas surface density, but ranges from low to
high ratios in regions of low surface density. The ratio correlates well with
the spatial distributions and intensities of FUV and IR emissions, with FUV
being the best correlated. It also correlates better with the ratio of IR
intensities (70/350mic), a proxy for dust temperature, than with the IR
intensities. Taken together, these results suggest either a direct or indirect
link between the dust heating by the interstellar radiation field and the
condition of GMCs, even though no efficient mechanism is known for a thermal
coupling of dust and bulk gas in GMCs.
