A New Look at the Galactic Circumnuclear Disk

• Published in: The Astrophysical journal Vol. 732; no. 2; pp. 120 - jQuery1323908884598='48'
• Main Authors: Oka, Tomoharu, Nagai, Makoto, Kamegai, Kazuhisa, Tanaka, Kunihiko
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 10.05.2011; IOP
• Subjects: Astronomy; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; Asymmetry; Bursting; CARBON COMPOUNDS; CARBON MONOXIDE; CARBON OXIDES; CHALCOGENIDES; Density; Disks; Earth, ocean, space; ELEMENTS; Exact sciences and technology; GALAXIES; HYDROCYANIC ACID; HYDROGEN; HYDROGEN COMPOUNDS; INORGANIC ACIDS; INORGANIC COMPOUNDS; NITRILES; NONMETALS; ORGANIC COMPOUNDS; ORGANIC NITROGEN COMPOUNDS; OXIDES; OXYGEN COMPOUNDS; Progenitors (astrophysics); SILICON COMPOUNDS; SILICON OXIDES; Star formation; Galaxies; Intensity; Accretion rate; Giant molecular cloud; S-parameters; ISM: kinematics and dynamics; Molecular gas; Galactic disks; Galaxy: center; Hydrogen cyanides; Star formation; radio lines: ISM; Dynamics; ISM: molecules; Dense gas; Kinematics; Galaxy nuclei; galaxies: nuclei; Diameter
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/732/2/120

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) X 105 M 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) 104.1 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*.