Cloud structures in M17 SWex : Possible cloud-cloud collision
  Using wide-field $^{13}$CO ($J=1-0$) data taken with the Nobeyama 45-m
telescope, we investigate cloud structures of the infrared dark cloud complex
in M17 with SCIMES. In total, we identified 118 clouds that contain 11 large
clouds with radii larger than 1 pc. The clouds are mainly distributed in the
two representative velocity ranges of 10 $-$ 20 km s$^{-1}$ and 30 $-$ 40 km
s$^{-1}$. By comparing with the ATLASGAL catalog, we found that the majority of
the $^{13}$CO clouds with 10 $-$ 20 km s$^{-1}$ and 30 $-$ 40 km s$^{-1}$ are
likely located at distances of 2 kpc (Sagittarius arm) and 3 kpc (Scutum arm),
respectively. Analyzing the spatial configuration of the identified clouds and
their velocity structures, we attempt to reveal the origin of the cloud
structure in this region. Here we discuss three possibilities: (1) overlapping
with different velocities, (2) cloud oscillation, and (3) cloud-cloud
collision. From the position-velocity diagrams, we found spatially-extended
faint emission between $\sim$ 20 km s$^{-1}$ and $\sim$ 35 km s$^{-1}$, which
is mainly distributed in the spatially-overlapped areas of the clouds. We also
found that in some areas where clouds with different velocities overlapped, the
magnetic field orientation changes abruptly. The distribution of the diffuse
emission in the position-position-velocity space and the bending magnetic
fields appear to favor the cloud-cloud collision scenario compared to other
scenarios. In the cloud-cloud collision scenario, we propose that two $\sim$35
km s$^{-1}$ foreground clouds are colliding with clouds at $\sim$20 km s$^{-1}$
with a relative velocity of 15 km s$^{-1}$. These clouds may be substructures
of two larger clouds having velocities of $\sim$ 35 km s$^{-1}$ ($\gtrsim 10^3
$ M$_{\odot}$) and $\sim$ 20 km s$^{-1}$ ($\gtrsim 10^4 $ M$_{\odot}$),
respectively.
