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Akihiko Hirota, Nario Kuno, Naoko Sato, Hiroyuki Nakanishi, Tomoka Tosaki, Kazuo Sorai, Variation of Molecular Gas Properties Across the Spiral Arms in IC 342: Large-Scale 13CO (1–0) Emission, Publications of the Astronomical Society of Japan, Volume 62, Issue 5, 25 October 2010, Pages 1261–1275, https://doi.org/10.1093/pasj/62.5.1261
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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.
