Variation of Molecular Gas Properties Across the Spiral Arms in IC 342: Large-Scale 13CO (1-0) Emission

We present a (CO)-C-13 (1-0) mapping survey of the nearby galaxy IC 342 carried out with the Nobeyama 45 m radio telescope. The map covers a 320 '' x 320 '' region, including its center, bar, and most of one of the spiral arms. The (CO)-C-13 (1-0) data were compared with (CO)-C-12 (1-0), H alpha, 24 mu m, and K-s-band images to investigate spatial relations among molecular clouds, star-forming regions, and the stellar potential. The line ratio between both CO lines, I[(CO)-C-13 (1-0)]/I[(CO)-C-12 (1-0)] equivalent to R-13/12, was also derived to investigate the variation of molecular gas properties. The mean value of R-13/12 in the center of IC 342 is low (similar to 0.10 +/- 0.01) compared to the disk region (similar to 0.15 +/- 0.01). While R-13/12 in the central region is rather uniform, significant variations of R-13/12 were found in the spiral arms; that is, while it is low (similar to 0.1) at both ends of the bar and at the giant molecular cloud association (GMA) found in the spiral arm, it is high (0.14-0.20) downstream from the low R-13/12 region. The low- and high-R-13/12 regions in the disk correspond to the (CO)-C-12 peak and star-forming regions, respectively. The low R-13/12 in the galactic center is likely due to the higher gas temperature in the starburst region. The most probable explanation of the R-13/12 variation in the disk is that a low R-13/12 in the GMA and the ends of the bar reflects an increased fraction of the diffuse molecular component, which has a low column density and low volume density. Around the GMA and the north end of the bar, not only are the star-forming regions downstream from the CO distributions, but R-13/12 is also higher downstream, implying the presence of gas compression and ionized/molecular gas spatial offsets, as predicted by density wave models.