The FUGIN CO survey 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 ^ { -1 } 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 { 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 ^ { -1 } , and the center velocity decreases by \sim 15 km s ^ { -1 } 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 } M _ { \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 .