Molecular clouds towards RCW 49 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 (hereafter Wd2), 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^{+ 1.1}_{-
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 Wd2 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.
