Molecular Gas Properties of the Starburst Nucleus of IC 342: High‐Resolution13CO (2–1) Imaging

We present a map of the J = 2-1 transition of 13 CO in the starburst nucleus of IC 342 made with the Owens Valley Millimeter Array. The interferometric 13 CO map allows us to directly compare with 12 CO (1-0), 12 CO (2-1), and 13 CO (1-0) maps of nearly identical (~4 5) resolution. While all four transitions show similar basic morphology, there are spatial differences between the 12 CO and 13 CO transitions that show up in ratio maps. In particular, the 13 CO (2-1)/ 13 CO (1-0) ratio has a markedly different distribution across the nuclear region than does 12 CO (2-1)/ 12 CO (1-0), indicating that 12 CO and 13 CO trace different components of the molecular gas. These differences are explained if 12 CO traces the warm, photodissociation region "skins" of the clouds and 13 CO traces the cooler, interior portions constituting the bulk of the molecular gas. We derive excitation temperatures for the bulk of the cloud mass of ~10-20 K and densities of ~10 3.7 cm -2 along the molecular minispiral.