INTERPRETING THE HCN CO INTENSITY RATIO IN THE GALACTIC-CENTER/

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 W ay, and images of HCN J = 3 --> 2 emission, which requires high densit ies for excitation (n(H2) greater than or similar to 10(6) cm(-3)), fr om Sgr A and Sgr B. The ratio of integrated HCN and CO J = 1 --> 0 int ensities is a sensitive measure of molecular gas pressure, and the rat io of integrated HCN J = 3 --> 2 and J = 1 --> 0 intensities uncovers density enhancements in the maps. However, the HCN/CO ratio is difficu lt to model without knowing the relative HCN and CO abundances. Furthe rmore, 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 Galact ic center is at high densities (n(H2) similar to 10(4) cm(-3)), roughl y an order of magnitude higher than cloud densities in the Galactic di sk. Most of the dense gas traced by HCN J = 3 --> 2 emission is coinci dent with star-forming regions and cloud interaction zones, and not ne cessarily 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 ext rapolate this result to other spirals, studies such as this are sensit ive to the average gas properties in galactic nuclei, despite poor spa tial resolution.