Galactic-Center Molecular Arms, Ring, and Expanding Shell. I. Kinematical Structures in Longitude--Velocity Diagrams

Analyzing the (l, b, V_LSR) data cube of (13) CO(J=1--0)-line emission from the Bell-Telephone-Laboratory survey, we have investigated the molecular gas distribution and kinematics in the central +/- 1(deg) (+/- 150 pc) region of the Galaxy. We have applied the pressing method to remove the local- and foreground-gas components at low velocities in order to estimate the intensity more quantitatively. Two major dense molecular arms have been identified in longitude--radial velocity (l, V)\ diagrams as apparently rigidly-rotating ridges. The ridges are spatially identified as two arms, which we call the Galactic-Center molecular Arms (GCA). The arms compose a rotating ring of radius 120 pc (the 120-pc Molecular Ring), whose inclination is i =~ 85(deg) . The Sgr B molecular complex is associated with GCA I, and Sgr C complex is located on GCA II. These arms are as thin as 13 to 15 pc, except for vertically extended massive complexes around Sgr B and C. The (l, V)\ behavior of the arms can be qualitatively reproduced by a model which assumes spiral arms of gas. Assuming a small pitch angle for the arms, we tried to deconvolve the (l, V) diagram to a projection on the galactic plane, and present a possible face-on CO map as seen from the galactic pole, which also reveals a molecular ring and arms. We have estimated the masses of these molecular features, using the most recent value of the CO-to-H_2\ conversion factor, taking into account its metallicity dependence and radial gradient in the Galaxy. The estimated molecular masses and kinetic energy are about a factor of three smaller than those reported in the literature using the conventional conversion factor.