Molecular gas in the compact H{\sc ii} region RCW 166; possible evidence for an early phase of cloud-cloud collision prior to the bubble formation

Abstract: Young HII regions are an important site to study O star formation based on distributions of ionized and molecular gas. We revealed that two molecular clouds at $\sim 48$ km s$^{-1}$ and $\sim 53$ km s$^{-1}$ are associated with the HII regions G018.149-00.283 in RCW 166 by using the JCMT CO High-Resolution Survey (COHRS) of the $^{12}$CO ($J$=3--2) emission. G018.149-00.283 comprises a bright ring at 8 $\mu$m and an extended HII region inside the ring. The $\sim 48$ km s$^{-1}$ cloud delineates the ring, and the $\sim 53$ km s$^{-1}$ cloud is located within the ring, indicating a complementary distribution between the two molecular components. We propose a hypothesis that high-mass stars within G018.149-00.283 were formed by triggering in cloud-cloud collision at a projected velocity separation of $\sim 5$ km s$^{-1}$. We argue that G018.149-00.283 is in an early evolutionary stage, $\sim 0.1$ Myr after the collision according to the scheme by [hab92] which will be followed by a bubble formation stage like RCW 120. We also suggested that nearby HII regions N21 and N22 are candidates for bubbles possibly formed by cloud-cloud collision. [ino13] showed that the interface gas becomes highly turbulent and realizes a high-mass accretion rate of $10^{-3}$ -- $10^{-4}$ $M_{\odot}$ $/$yr by magnetohydrodynamical numerical simulations, which offers an explanation of the O-star formation. A fairly high frequency of cloud-cloud collision in RCW 166 is probably due to the high cloud density in this part of the Scutum arm.