Interpreting the HCN/CO Intensity Ratio in the Galactic Center

Abstract

We study the dense molecular gas in the Galactic center using maps of HCN and CO J = 1 → 0 emission from the central 630 pc of the Milky Way, and images of HCN J = 3 → 2 emission, which requires high densities for excitation (nH2 106 cm-3), from Sgr A and Sgr B. The ratio of integrated HCN and CO J = 1 → 0 intensities is a sensitive measure of molecular gas pressure, and the ratio of integrated HCN J = 3 → 2 and J = 1 → 0 intensities uncovers density enhancements in the maps. However, the HCN/CO ratio is difficult to model without knowing the relative HCN and CO abundances. Furthermore, because of the different filling factors of HCN and CO emission, models that use homogeneous clouds may not be accurate for analyzing the HCN/CO ratio. Most of the mass traced by HCN and CO in the Galactic center is at high densities (nH2 ~ 104 cm-3), roughly an order of magnitude higher than cloud densities in the Galactic disk. Most of the dense gas traced by HCN J = 3 → 2 emission is coincident with star-forming regions and cloud interaction zones, and not necessarily with emission peaks. We smoothed the HCN and CO maps to the typical spatial resolution of extragalactic observations, and repeated the analysis. The single large-scale measurement was sensitive to the mass-weighted average properties of the map. Therefore, if we can extrapolate this result to other spirals, studies such as this are sensitive to the average gas properties in galactic nuclei, despite poor spatial resolution.