FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m
  telescope (FUGIN) 2: Possible evidence for formation of NGC~6618 cluster in
  M17 by cloud-cloud collision
  We present $^{12}$CO $J=$1--0, $^{13}$CO $J=$1--0 and C$^{18}$O $J=$1--0
images of the M17 giant molecular clouds obtained as part of FUGIN (FOREST
Ultra-wide Galactic Plane Survey InNobeyama) project. The observations cover
the entire area of M17 SW and M17 N clouds at the highest angular resolution
($\sim$19$"$) to date which corresponds to $\sim$ 0.15 pc at the distance of
2.0 kpc. We find that the region consists of four different velocity
components: very low velocity (VLV) clump, low velocity component (LVC), main
velocity component (MVC), and high velocity component (HVC). The LVC and the
HVC have cavities. UV photons radiated from NGC 6618 cluster penetrate into the
N cloud up to $\sim$ 5 pc through the cavities and interact with molecular gas.
This interaction is correlated with the distribution of YSOs in the N cloud.
The LVC and the HVC are distributed complementary after that the HVC is
displaced by 0.8 pc toward the east-southeast direction, suggesting that
collision of the LVC and the HVC create the cavities in both clouds. The
collision velocity and timescale are estimated to be 9.9 km s$^{-1}$ and $1.1
\times 10^{5}$ yr, respectively. The high collision velocity can provide the
mass accretion rate up to 10$^{-3}$ $M_{\solar}$ yr$^{-1}$, and the high column
density ($4 \times 10^{23}$ cm$^{-2}$) might result in massive cluster
formation. The scenario of cloud-cloud collision likely well explains the
stellar population and its formation history of NGC 6618 cluster proposed by
Hoffmeister et al. (2008).
