A LARGE-SCALE CLOUD COLLISION IN THE GALACTIC-CENTER MOLECULAR CLOUD NEAR SAGITTARIUS B2

We present high-resolution images of the (CO)-C-13 (J = 1-0) emission from the molecular cloud near the Sgr B2 star-forming complex taken wi th the Nobeyama 45 m telescope. The data, supplemented by the larger m aps taken with the Bell Labs 7 m telescope by Bally et al., show three characteristic kinematical features; (1) a ''Shell'' of emission seen at V(LSR) = 20-40 km s-1, (2) close morphological correlation between a depression in emission or a ''Hole'' at V(LSR) = 40-50 km s-1 and a well-defined emission region or a '' Clump '' at V(LSR) = 70-80 km s- 1, and (3) a straight ridge of emission or an ''Edge'' seen at V(LSR) = 50-70 km s-1. The appearance and relative location of the Shell, the Hole, and the Clump are similar to those associated with a cloud coll ision event visualized by numerical simulations. We propose a scenario in which a dense, massive (10(6) M.) cloud (Clump) approximately 10 p c in diameter has collided, at a relative velocity of approximately 30 km s-1, with the extended less dense gas of the molecular cloud compl ex in the Galactic center region, and has created a hole with a bow sh ock (Hole and Shell). In the course of this collision, dense and massi ve molecular cloud cores may have formed at the interface between the colliding clouds, and their collapse resulted in the current burst of massive star formation in the Sgr B2 complex. This is the clearest obs erved case of a collision event between giant molecular clouds that tr iggers intense formation of massive (O-type) stars.