Sensitive CO Observations of the LMC Supergiant Shells with NANTEN: Their Effects on the Formation of Molecular Clouds and Stellar Clusters

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Abstract A statistical study of the effects of supergiant shells (SGSs) on the formation of stars and molecular clouds was made based on sensitive CO ($J=1 \hbox{--} 0$) observations of the molecular clouds in the LMC with NANTEN. The sensitive observations have detected 168 CO clouds whose mass ranges from $\sim 4 \times 10^4 \,{{{M}_{\odot}}}$ to $3 \times 10^6 \,{{{M}_{\odot}}}$. Out of the 168 clouds, 63 are associated with the SGSs, accounting for $\gtrsim 35\%$. The surface number and mass densities of the CO clouds are higher by a factor of 1.5–2 at the edge of the SGSs than elsewhere. This suggests the formation of CO clouds under the dynamical effects of the SGSs, such as the accumulation of interstellar medium. Young stellar clusters are more actively formed on the side of the CO clouds facing to the center of the SGSs, particularly where the $\mathrm{H}\alpha$ filaments are adjacent to the CO clouds. The number of clusters is increased by a factor of 2 on the side facing to the center of the SGSs with respect to the CO peaks compared with on the side far from the center. These results strongly suggest that cluster formation is triggered by dynamical effects of the SGSs. We find that $\sim 60\%$ of the young clusters are located within the boundary of the SGSs, and propose that a few 10% of the young clusters of $\tau \lt 10 \,\mathrm{Myr}$ have likely been formed due to dynamical effects of the SGSs.