A Large-Scale Cloud Collision in the Galactic Center Molecular Cloud Near Sagittarius B2

We present high-resolution images of the (13)CO(J = 1 to 0) emission from the molecular cloud near the Sgr B2 star-forming complex taken with the Nobeyama 45 m telescope. The data, supplemented by the larger maps 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 to 40 km/sec, (2) close morphological correlation between a depression in emission or a 'Hole' at V_LSR = 40 to 50 km/sec and a well-defined emission region or a 'Clump' at V_LSR = 70 to 80 km/sec, and (3) a straight ridge of emission or an 'Edge' seen at V_LSR = 50 to 70 km/sec. The appearance and relative location of the Shell, the Hole, and the Clump are similar to those associated with a cloud collision event visualized by numerical simulations. We propose a scenario in which a dense, massive (10⁶ solar mass) cloud (Clump) approximately 10 pc in diameter has collided, at a relative velocity of approximately 30 km/sec, with the extended less dense gas of the molecular cloud complex in the Galactic center region, and has created a hole with a bow shock (Hole and Shell). In the course of this collision, dense and massive 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 observed case of a collision event between giant molecular clouds that triggers intense formation of massive (O-ty pe) stars.