CO observations of the molecular gas in the galactic HII region Sh2-48;
  Evidence for cloud-cloud collision as a trigger of high-mass star formation
  Sh2-48 is a Galactic HII region located at 3.8 kpc with an O9.5-type star
identified at its center. As a part of the FOREST Unbiased Galactic plane
Imaging survey using the Nobeyama 45-m telescope (FUGIN) project, we obtained
the CO J=1-0 dataset for a large area of Sh2-48 at a spatial resolution of
21"(~0.4 pc), which we used to find a molecular cloud with a total molecular
mass of ~3.8x10^4 Mo associated with Sh2-48. The molecular cloud has a
systematic velocity shift within a velocity range ~42-47 km/s . On the lower
velocity side the CO emission spatially corresponds with the bright 8 {\mu}m
filament at the western rim of Sh2-48, while the CO emission at higher
velocities is separated at the eastern and western sides of the 8{\mu}m
filament. This velocity change forms V-shaped, east-west-oriented feature on
the position-velocity diagram. We found that these lower and higher-velocity
components are, unlike the infrared and radio continuum data, physically
associated with Sh2-48. To interpret the observed V-shaped velocity
distribution, we assessed a cloud-cloud collision scenario and found from a
comparison between the observations and simulations that the velocity
distribution is an expected outcome of a collision between a cylindrical cloud
and a spherical cloud, with the cylindrical cloud corresponding to the
lower-velocity component, and the two separated components in the
higher-velocity part interpretable as the collision-broken remnants of the
spherical cloud. Based on the consistency of the ~1.3Myr estimated formation
timescale of the HII region with that of the collision, we concluded that the
high-mass star formation in Sh2-48 was triggered by the collision.
