A New Look at the Galactic Circumnuclear Disk

We report the results of millimeter and submillimeter molecular line mapping observations of the Galactic circumnuclear disk (CND). The CND appears as a large, asymmetric disk of warm molecular gas with a high CO J = 3-2/CO J = 1-0 intensity ratio exceeding 1.5. It has a mass of (2-5) × 10⁵ M _sun and a diameter of about 10 pc, including a well-known 2-pc radius ring of dense molecular gas around the minispiral. The CND can be clearly traced by the J = 1-0 lines of HCN, H¹³CN, HCO⁺, and HNC, but it is barely traced by N₂H⁺, SiO, CCS, and HC₃N lines. These data confirm the entity of the CND, and the 2-pc ring is just a part of it. Line ratios suggest that the CND is chemically immature, having higher density and higher temperature than the ambient gas. A one-zone large-velocity-gradient analysis finds that molecular gas in the CND has a typical kinetic temperature of T _k ~= 63 K and H₂ density of n(H₂) ~= 10^4.1 cm^-3. The bulk of the CND seems to have an overall, systematic infall motion, with a velocity of V _infall ~= 50 km s^-1. Our results are consistent with the scenario that the CND has been formed by tidal capture and disruption of a giant molecular cloud (GMC). The progenitor GMC may have been formed recently inside the 120-pc ring, possibly by the encounter with the small inner bar of the Galaxy. Toomre's Q parameter indicates that the CND is gravitationally stable now, but it will become unstable and fragment as gas accumulates at r ~= 2 pc. It would trigger a burst of star formation, and subsequent processes could enhance the mass accretion rate to Sgr A*.