FUGIN: Molecular Gas in Spitzer Bubble N4—Possible Evidence for a Cloud-Cloud Collision as a Trigger of Massive Star Formations

Herein, we present ¹²CO (J = 1-0) and ¹³CO (J = 1-0) emission-line observations via the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN) toward Spitzer bubble N4. We observed clouds at three discrete velocities: 16, 19, and 25 km s^-1. Their masses were 0.1 × 10⁴ M _⊙, 0.3 × 10⁴ M _⊙, and 1.4 × 10⁴ M _⊙, respectively. The distribution of the 25 km s^-1 cloud likely traces the ring-like structure observed at mid-infrared wavelength. The 16 and 19 km s^-1 clouds have not been recognized in previous observations of molecular lines. We could not find clear expanding motion of the molecular gas in N4. On the contrary, we found a bridge feature and a complementary distribution, which are discussed as observational signatures of a cloud-cloud collision (CCC), between the 16 and 25 km s^-1 clouds. We proposed a possible scenario wherein the formation of a massive star in N4 was triggered by a collision between the two clouds. The timescale of collision is estimated to be 0.2-0.3 Myr, which is comparable to the estimated dynamical age of the H II region of ∼0.4 Myr. In N4W, a star-forming clump located west of N4, we observed molecular outflows from young stellar objects and the observational signature of a CCC. Thus, we also proposed a possible scenario in which massive- or intermediate-mass star formation was triggered via a CCC in N4W.