Molecular clouds towards RCW 49 and Westerlund 2; Evidence for cluster formation triggered by cloud-cloud collision

TL;DR

This study presents CO observations of RCW 49 and Westerlund 2, revealing two molecular clouds likely involved in a cloud-cloud collision that triggered cluster formation around 4 million years ago.

Contribution

It provides new evidence linking cloud-cloud collision to star cluster formation, supported by detailed molecular gas observations and a revised distance estimate.

Findings

Two molecular clouds are spatially and kinematically associated with RCW 49.

A cloud-cloud collision approximately 4 million years ago likely triggered cluster formation.

The estimated distance to RCW 49 and Wd2 is about 5.4 kpc.

Abstract

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.