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

TL;DR

This study investigates the molecular gas distribution and kinematics in the Galactic Center region, identifying two dense molecular arms forming a rotating ring, and provides a face-on view of the molecular structure with revised mass estimates.

Contribution

It reveals the detailed structure of molecular arms and a ring in the Galactic Center, using a novel method to decompose velocity data and updating mass estimates with a metallicity-dependent conversion factor.

Findings

Identification of two dense molecular arms as rigidly rotating ridges.

Estimation of a 120-pc radius molecular ring with high inclination.

Revised molecular mass and energy estimates lower than previous values.

Abstract

Analyzing the (l, b, Vlsr) data cube of 13CO(J=1-0) line emission obtained by Bally et al, 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 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 about 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 LV 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 LV 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 masses of these molecular features, using the most recent value of the CO-to-H2 conversion factor taking into account its metallicity dependency 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.