HCN AND CO IN THE CENTRAL 630 PARSECS OF THE GALAXY

Using the QUARRY focal plane array at the FCRAO 14 m, we have mapped t he central 630 x 75 pc (l x b = 4.degrees 3 x 0.degrees 5) of the Gala xy at similar to 50 '' (2 pc) resolution in both CO and HCNJ = 1 --> 0 emission. Since HCN emission traces gas with densities greater than o r similar to 100 times larger than that traced by CO, the HCN luminosi ty measures the mass of dense (n greater than or similar to 10(5) cm(- 3)) molecular gas, and the HCN/CO intensity ratio measures gas density . The densest molecular gas is found in the molecular clouds associate d with the l = 1.degrees 5 complex, Sgr A, Sgr B, and Sgr C. When comp ared with the Milky Way at the same spatial resolution, the HCN lumino sities in the central kiloparsec of six nearby starburst galactic nucl ei are typically larger (factors of similar to 0.5-20), but the CO lum inosities are only moderately larger (factors of similar to 0.5-5). Av eraged over the central 200 pc, the HCN/CO intensity ratio in the star burst galaxies is larger than in the Milky Way, but a few starburst nu clei have similar or smaller HCN/CO intensity ratios when averaged ove r larger regions. The spatial extent of elevated HCN/CO ratios (greate r than or similar to 0.08) typically encompasses greater than or simil ar to 200 pc in the starburst galaxies but only similar to 50 pc in th e Milky Way. These results suggest that starburst nuclei have larger m asses of dense gas, higher average gas densities in the central 200 pc , and larger regions of enhanced gas density than the Galactic center. When averaged to 630 pc resolution, the HCN and CO spectra show two m ain velocity components: (1) a broad feature of width similar to 250 k m s(-1) centered on V-LSR = 0 km s(-1) and (2) a narrow feature of wid th similar to 90 km s(-1) centered on V-LSR = 50 km s(-1). For the bro ad component, the HCN/CO J = 1 --> 0 intensity ratio is nearly constan t over the entire velocity range, and its small value (R similar to 0. 03) is typical of that found in normal galaxies at several kiloparsecs spatial resolution. The gas in the broad velocity component is spatia lly extended and may represent diffuse molecular gas bound not to indi vidual clouds but rather directly to the Galactic center potential wel l in a central disk or bar. Gas in the narrow velocity component is as sociated with the dense giant molecular clouds associated with the l = 1.degrees 5 complex, Sgr A, B, and C.