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 1.2 ± 0.1 across the whole ∼1 kpc bar. This indicates that the HCN(1–0) and HCO⁺(1–0) lines have intermediate optical depth, and that the corresponding n_(H₂) of the gas producing the emission is of order 10^4.5−6 cm⁻³. We show that HCO⁺(1–0) is thermalized and HCN(1–0) is close to thermalization. The very tight correlation between the 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 an angular offset (∼10'') between the spatial distribution of molecular gas and the star formation sites. Finally, we find a breakdown of the L_IR–L_(HCN) 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 L_IR–L_(HCN) 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 a scale of 340 pc.

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