FOREST Unbiased Galactic Plane Imaging Survey with the Nobeyama 45-m
  Telescope (FUGIN) IV: Galactic Shock Wave and Molecular Bow Shock in the
  4-kpc Arm of the Galaxy
  The FUGIN CO survey with the Nobeyama 45-m Telescope revealed the 3D
structure of a galactic shock wave in the tangential direction of the 4-kpc
molecular arm. The shock front is located at G30.5+00.0+95 km/s on the
up-stream (lower longitude) side of the star-forming complex W43 (G30.8-0.03),
and composes a molecular bow shock (MBS) concave to W43, exhibiting an
arc-shaped molecular ridge perpendicular to the galactic plane with width $\sim
0^\circ.1$ (10 pc) and vertical length $\sim 1^\circ \ (100\ {\rm pc})$. The
MBS is coincident with the radio continuum bow of thermal origin, indicating
association of ionized gas and similarity to a cometary bright-rimmed cloud.
The up-stream edge of the bow is sharp with a growth width of $\sim 0.5$ pc
indicative of shock front property. The velocity width is $\sim 10$ km/s, and
the center velocity decreases by $\sim 15$ \kms from bottom to top of the bow.
The total mass of molecular gas in MBS is estimated to be $\sim 1.2\times 10^6
<_\odot$ and ionized gas $\sim 2\times 10^4 M_\odot$. The vertical disk
thickness increases step like at the MBS by $\sim 2$ times from lower to upper
longitude, which indicates hydraulic-jump in the gaseous disk. We argue that
the MBS was formed by the galactic shock compression of an accelerated flow in
the spiral-arm potential encountering the W43 molecular complex. A bow-shock
theory can well reproduce the bow morphology. We argue that molecular bows are
common in galactic shock waves not only in the Galaxy but also in galaxies,
where MBS are associated with giant cometary HII regions. We also analyzed the
HI data in the same region to obtain a map of HI optical depth and molecular
fraction. We found a firm evidence of HI-to-H$_{2}$ transition in the galactic
shock as revealed by a sharp molecular front at the MBS front.
