GBT/Argus Observations of Molecular Gas in the Inner Regions of IC 342
Abstract
We report observations of the ground state transitions of 12CO, 13CO C18O, HCN, and HCO+ at 88-115 GHz in the inner region of the nearby galaxy IC 342. These data were obtained with the 16-pixel spectroscopic focal plane array Argus on the 100-m Robert C. Byrd Green Bank Telescope (GBT) at 6-9 resolution. In the nuclear bar region, the intensity distributions of 12CO(1-0) and 13CO(1-0) emission trace moderate densities, and differ from the dense gas distributions sampled in C18O(1-0), HCN(1-0), and HCO+(1-0). We observe a constant HCN(1-0)-to-HCO+(1-0) ratio of 1.20.1 across the whole 1 kpc bar. This indicates that HCN(1-0) and HCO+(1-0) lines have intermediate optical depth, and that the corresponding nH2 of the gas producing the emission is of 104.5-6 cm-3. We show that HCO+(1-0) is thermalized and HCN(1-0) is close to thermalization. The very tight correlation between HCN(1-0) and HCO+(1-0) intensities across the 1~kpc bar suggests that this ratio is more sensitive to the relative abundance of the two species than to the gas density. We confirm the angular offset (10) between the spatial distribution of molecular gas and the star formation sites. Finally, we find a breakdown of the LIR-LHCN correlation at high spatial resolution due to the effect of incomplete sampling of star-forming regions by HCN emission in IC 342. The scatter of the LIR-LHCN relation decreases as the spatial scale increases from 10~to 30 (170-510~pc), and is comparable to the scatter of the global relation at the scale of 340 pc.
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