Molecular clouds in the supergiant shell LMC 4: Evidence for triggered cluster formation

We have completed fully sampled observations of CO (J = 1-0) toward the supergiant shell (SGS) LMC 4 at a linear resolution of 40 pc with NANTEN. The result is the discovery of 18 clumpy CO clouds, 17 of which well delineate the SGS traced by H alpha and H I. The remaining one cloud inside the SGS exhibits a cometary shape with an H I tail heading toward the center of the SGS. A comparison between the CO clouds and the young stars, including H ii regions and stellar clusters, indicates that all the associated young stars are on the side of the clouds facing toward the center of the SGS. The typical separation between the cloud peaks and young stars is similar to 40-100 pc. These facts strongly suggest that star formation has been triggered by the expanding shell that is driven by OB stars, indicating the important role of SGSs in cluster formation in the LMC.