Molecular Clouds Towards RCW49 and Westerlund 2; Evidence for Cluster Formation Triggered by Cloud-Cloud Collision

We have made CO(J = 2− 1) observations towards the HII region RCW 49 and its ionizing source, the rich stellar cluster Westerlund 2, with the NANTEN2 sub-mm telescope. These observations have revealed that two molecular clouds in velocity ranges of −11 to +9 km s and 11 to 21 km s respectively, show remarkably good spatial correlations with the Spitzer IRAC mid-infrared image of RCW 49, as well a velocity structures indicative of localized expansion around the bright central regions and stellar cluster. This strongly argues that the two clouds are physically associated with RCW 49. We obtain a new kinematic distance estimate to RCW 49 and Wd2 of 5.4 −1.4 kpc, based on the mean velocity and velocity spread of the associated gas. We argue that acceleration of the gas by stellar winds from Westerlund 2 is insufficient to explain the entire observed velocity dispersion of the molecular gas, and suggest a scenario in which a collision between the two clouds ∼4 Myrs ago may have triggered the formation of the stellar cluster. Subject headings: ISM: clouds — open clusters and associations: individual (Westerlund 2) — HII regions: individual (RCW 49)