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

Abstract

Abstract We present a $^{13}$CO (1–0) mapping survey of the nearby galaxy IC 342 carried out with the Nobeyama 45 m radio telescope. The map covers a 320$^{\prime\prime} \times$ 320$^{\prime\prime}$ region, including its center, bar, and most of one of the spiral arms. The $^{13}$CO (1–0) data were compared with $^{12}$CO (1–0), H$\alpha$, 24$\ \mu$m, and $K_{\rm s}$-band images to investigate spatial relations among molecular clouds, star-forming regions, and the stellar potential. The line ratio between both CO lines, $I$[$^{13}$CO (1–0)]$/I$[$^{12}$CO (1–0)] $\equiv R_{13/12}$, 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 ($\sim\ $0.10$\ \pm\ $0.01) compared to the disk region ($\sim\ $0.15$\ \pm\ $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 ($\sim\ $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 $^{12}$CO 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.