RCW 120 is a Galactic H ii region having a beautiful ring shape bright in infrared .
Our new CO J =1–0 and J =3–2 observations performed with the NANTEN2 , Mopra , and ASTE telescopes have revealed that two molecular clouds with a velocity separation of 20 km s ^ { -1 } are both physically associated with RCW 120 .
The cloud at -8 km s ^ { -1 } apparently traces the infrared ring , while the other cloud at -28 km s ^ { -1 } is distributed just outside the opening of the infrared ring , interacting with the H ii region as supported by high kinetic temperature of the molecular gas and by the complementary distribution with the ionized gas .
A spherically expanding shell driven by the H ii region is usually discussed as the origin of the observed ring structure in RCW 120 .
Our observations , however , indicate no evidence of the expanding motion in the velocity space , being inconsistent with the expanding shell model .
We here postulate an alternative that , by applying the model introduced by Habe & Ohta ( 29 ) , the exciting O star in RCW 120 was formed by a collision between the present two clouds at a colliding velocity \sim 30 km s ^ { -1 } .
In the model , the observed infrared ring can be interpreted as the cavity created in the larger cloud by the collision , whose inner surface is illuminated by the strong UV radiation after the birth of the O star .
We discuss that the present cloud-cloud collision scenario explains the observed signatures of RCW 120 , i.e. , its ring morphology , coexistence of the two clouds and their large velocity separation , and absence of the expanding motion .