Bow-shock structure of Sgr-B molecular-cloud complex in the Galactic Centre inferred from 3D CO-line kinematics

Three-dimensional (3D) bubble structure of the Sgr-B molecular-cloud complex is derived by a kinematical analysis of CO-line archival cube data of the Galactic Centre (GC) observed with the Nobeyama 45-m telescope. The line-of-sight depth is estimated by applying the face-on transformation method of radial velocity to the projected distance on the Galactic plane considering the Galactic rotation of the central molecular zone (CMZ). The 3D complex exhibits a conical-horn structure with the Sgr-B2 cloud located in the farthest end on the line of sight at radial velocity vₗₛᵣ ~ 70 km s⁻¹, and the entire complex composes a lopsided bubble opening toward the Sun at vₗₛᵣ~ 50 to 30 km s⁻¹. The line-of-sight extent of the complex is ~ 100 pc according to the large velocity extent for several tens of km s⁻¹ from Sgr-B2 to the outskirts. The entire complex exhibits a flattened conical bubble with full sizes ~ 40 \ pc × 20 \ pc × 100 \ pc in the l, b and line-of-sight directions, respectively. Based on the 3D analysis, we propose a formation scenario of the giant molecular bubble structure due to a galactic bow shock, and suggest that the star formation in Sgr-B2 was enhanced by dual-side compression (DSC) of the B2 cloud by the Galactic shock wave from up-stream and expanding H ii region from the down-stream side of the GC Arm I in Galactic rotation.