Detailed CO ($J$ = 1--0, 2--1 and 3--2) observations toward an H{sc ii} region RCW~32 in the Vela Molecular Ridge

We made CO ($J$ = 1--0, 2--1, and 3--2) observations toward an H{sc ii} region RCW~32 in the Vela Molecular Ridge. The CO gas distribution associated with the H{sc ii} region was revealed for the first time at a high resolution of 22 arcsec. The results revealed three distinct velocity components which show correspondence with the optical dark lanes and/or H$alpha$ distribution. Two of the components show complementary spatial distribution which suggests collisional interaction between them at a relative velocity of $sim$4 km~s$^{-1}$. Based on these results, we present a hypothesis that cloud-cloud collision determined the cloud distribution and triggered formation of the exciting star ionizing RCW~32. The collision time scale is estimated from the cloud size and the velocity separation to be $sim$2 Myrs and the collision terminated $sim$1 Myr ago, which is consistent with an age of the exciting star and the associated cluster. By combing the previous works on the H{sc ii} regions in the Vela Molecular Ridge, we argue that the majority, at least four, of the H{sc ii} regions in the Ridge were formed by triggering of cloud-cloud collision.