FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN): Molecular clouds toward W 33; possible evidence for a cloud–cloud collision triggering O star formation

We observed molecular clouds in the W33 high-mass star-forming region associated with compact and extended HII regions using the NANTEN2 telescope as well as the Nobeyama 45-m telescope in the $J=$1-0 transitions of $^{12}$CO, $^{13}$CO, and C$^{18}$O as a part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) legacy survey. We detected three velocity components at 35 km s$^{-1}$, 45 km s$^{-1}$, and 58 km s$^{-1}$. The 35 km s$^{-1}$ and 58 km s$^{-1}$ clouds are likely to be physically associated with W33 because of the enhanced $^{12}$CO $J=$ 3-2 to $J=$1-0 intensity ratio as $R_{\rm 3-2/1-0} > 1.0$ due to the ultraviolet irradiation by OB stars, and morphological correspondence between the distributions of molecular gas and the infrared and radio continuum emissions excited by high-mass stars. The two clouds show complementary distributions around W33. The velocity separation is too large to be gravitationally bound, and yet not explained by expanding motion by stellar feedback. Therefore, we discuss that a cloud-cloud collision scenario likely explains the high-mass star formation in W33.