FOREST unbiased Galactic plane imaging survey with the Nobeyama 45 m
  telescope (FUGIN): Possible evidence of cloud-cloud collisions triggering
  high-mass star formation in the giant molecular cloud M16 (Eagle Nebula)
  M16, the Eagle Nebula, is an outstanding \HII \ region which exhibits
extensive high-mass star formation and hosts remarkable "pillars". We herein
obtained new $^{12}$CO $J=$1-0 data for the region observed with NANTEN2, which
were combined with the $^{12}$CO $J=$1-0 data obtained using FUGIN survey.
These observations revealed that a giant molecular cloud (GMC) of $\sim 1.3
\times 10^5$ \Msun \ is associated with M16, which is elongated by over 30 pc
and is perpendicular to the galactic plane, at a distance of 1.8 kpc. This GMC
can be divided into the northern (N) cloud, the eastern (E) filament, the
southeast (SE) cloud, the southeast (SE) filament, and the southern (S) cloud.
We also found two velocity components (blue and red shifted component) in the N
cloud. The blue-shifted component shows a ring-like structure, as well as the
red-shifted component coincides with the intensity depression of the ring-like
structure. The position-velocity diagram of the components showed a V-shaped
velocity feature. The spatial and velocity structures of the cloud indicated
that two different velocity components collided with each other at a relative
velocity of 11.6 \kms. The timescale of the collision was estimated to be $\sim
4 \times 10^5$ yr. The collision event reasonably explains the formation of the
O9V star ALS15348, as well as the shape of the Spitzer bubble N19. A similar
velocity structure was found in the SE cloud, which is associated with the
O7.5V star HD168504. In addition, the complementary distributions of the two
velocity components found in the entire GMC suggested that the collision event
occurred globally. On the basis of the above results, we herein propose a
hypothesis that the collision between the two components occurred sequentially
over the last several $10^{6}$ yr and triggered the formation of O-type stars
in the NGC6611 cluster.
