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

Abstract: 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 $v_{\rm lsr} \sim 70$ km s$^{-1}$, and the entire complex composes a lopsided bubble opening toward the Sun at $v_{\rm lsr}\sim 50$ to 30 km s$^{-1}$. The line-of-sight extent of the complex is $\sim 100$ 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 $\sim 40 {\rm pc} \times 20 {\rm pc} \times 100 {\rm pc}$ 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 of the B2 cloud by the Galactic shock wave from up-stream and expanding HII region from the down-stream side of the GC Arm I in Galactic rotation.