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The HII region W3 is one of the most outstanding regions of high-mass starformation. Based on a new analysis of the $^{12}$CO($J$ = 2-1) data obtained at38$"$ resolution, we have found that each of the two active regions ofhigh-mass star formation, W3 Main and W3(OH), is associated with two clouds ofdifferent velocities separated by 3-4 km s$^{-1}$, having cloud mass of2000-4000 $M_\odot$ in each. In the two regions we have found typicalsignatures of a cloud-cloud collision, i.e.,the complementary distributionwith/without a displacement between the two clouds and/or a V-shape in theposition-velocity diagram. We frame a hypothesis that a cloud-cloud collisiontriggered the high-mass star formation in the two regions. The collision in W3Main involves a small cloud of $\sim$5 pc in diameter which collided with alarge cloud of 10 pc $\times$ 20 pc. The collision in W3 Main compressed thegas in the direction of the collision path toward the west over a timescale of$\sim$1 Myr, where the dense gas W3 core associated with ten O stars areformed. The collision also produced a cavity in the large cloud having a sizesimilar to the small cloud. The collision in W3(OH) has a younger timescale of$\sim$0.5 Myr and the forming-star candidates are heavily embedded in theclouds. The results reinforce the idea that a cloud-cloud collision is anessential process in high-mass star formation by rapidly creating the initialcondition of 1 g cm$^{-2}$ in the natal gas.

A Kinematic Analysis of the Giant Molecular Complex W3; Possible  Evidence for Cloud-Cloud Collisions that Triggered OB Star Clusters in W3  Main and W3(OH)