Molecular clouds in the NGC 6334 and NGC 6357 region; Evidence for a 100 pc-scale cloud-cloud collision triggering the Galactic mini-starbursts

Abstract

We carried out new CO ($J=$1-0, 2-1 and 3-2) observations with NANTEN2 and ASTE in the region of the twin Galactic mini-starbursts NGC 6334 and NGC 6357. We detected two velocity molecular components of 12 km s$^{-1}$ velocity separation, which is continuous over 3 degrees along the plane. In NGC 6334 the two components show similar two-peaked intensity distributions toward the young HII regions and are linked by a bridge feature. In NGC 6357 we found spatially complementary distribution between the two velocity components as well as a bridge feature in velocity. Based on these results we hypothesize that the two clouds in the two regions collided with each other in the past few Myr and triggered formation of the starbursts over $\sim$ 100 pc. We suggest that the formation of the starbursts happened toward the collisional region of $\sim$ 10-pc extents with initial high molecular column densities. For NGC 6334 we present a scenario which includes spatial variation of the colliding epoch due to non-uniform cloud separation. The scenario possibly explains the apparent age difference among the young O stars in NGC 6334 raging from $10^4$ yrs to $10^6$ yrs; the latest collision happened within $10^5$ yrs toward the youngest stars in NGC 6334 I(N) and I which exhibit molecular outflows without HII regions. For NGC 6357 the O stars were formed a few Myrs ago, and the cloud dispersal by the O stars is significant. We conclude that cloud-cloud collision offers a possible explanation of the min-starburst over a 100-pc scale.