Young H ii 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 H ii 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 H ii 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 \citet hab92 which will be followed by a bubble formation stage like RCW 120 .
We also suggested that nearby H ii regions N21 and N22 are candidates for bubbles possibly formed by cloud-cloud collision . \citet 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 .