Molecular gas in a Spitzer bubble N4: possible evidence for cloud-cloud
  collisions as a trigger of massive star formation
  Herein, we present the 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 a Spitzer bubble N4. We observed clouds
of three discrete velocities: 16, 19, and 25 km/s. Their masses were 0.1x10^4
Msun, 0.3x10^4 Msun, and 1.4x10^4 Msun, respectively. The distribution of the
25-km/s cloud likely traces the ring-like structure observed at mid-infrared
wavelength. 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, between the 16- and 25-km/s clouds. We proposed a possible scenario
wherein the formation of a massive star in N4 was triggered by a collision
between the two clouds; however whereas the 19-km/s cloud is possibly not a
part of the interaction with N4. The time scale of collision is estimated to be
0.2-0.3 Myr, which is comparable to the estimated dynamical age of the HII
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 cloud-cloud collision. Thus, we also proposed a possible
scenario in which massive- or intermediate-mass star formation was triggered
via a cloud-cloud collision in N4W.
