The Two Molecular Clouds in RCW 38: Evidence for the Formation of the Youngest Super Star Cluster in the Milky Way Triggered by Cloud-Cloud Collision

We present distributions of two molecular clouds having velocities of 2 and 14 km s^-1 toward RCW 38, the youngest super star cluster in the Milky Way, in the ¹²CO J = 1-0 and 3-2 and ¹³CO J = 1-0 transitions. The two clouds are likely physically associated with the cluster as verified by the high intensity ratio of the J = 3-2 emission to the J = 1-0 emission, the bridging feature connecting the two clouds in velocity, and their morphological correspondence with the infrared dust emission. The velocity difference is too large for the clouds to be gravitationally bound. We frame a hypothesis that the two clouds are colliding with each other by chance to trigger formation of the ∼20 O stars that are localized within ∼0.5 pc of the cluster center in the 2 km s^-1 cloud. We suggest that the collision is currently continuing toward part of the 2 km s^-1 cloud where the bridging feature is localized. This is the third super star cluster alongside Westerlund 2 and NGC 3603 where cloud-cloud collision has triggered the cluster formation. RCW 38 is the youngest super star cluster in the Milky Way, holding a possible sign of on-going O star formation, and is a promising site where we may be able to witness the moment of O star formation.