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The FUGIN CO survey with the Nobeyama 45-m Telescope revealed the 3Dstructure of a galactic shock wave in the tangential direction of the 4-kpcmolecular arm. The shock front is located at G30.5+00.0+95 km/s on theup-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 anarc-shaped molecular ridge perpendicular to the galactic plane with width $\sim0^\circ.1$ (10 pc) and vertical length $\sim 1^\circ \ (100\ {\rm pc})$. TheMBS is coincident with the radio continuum bow of thermal origin, indicatingassociation 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 $\sim 0.5$ pcindicative of shock front property. The velocity width is $\sim 10$ km/s, andthe 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 diskthickness increases step like at the MBS by $\sim 2$ times from lower to upperlongitude, which indicates hydraulic-jump in the gaseous disk. We argue thatthe MBS was formed by the galactic shock compression of an accelerated flow inthe spiral-arm potential encountering the W43 molecular complex. A bow-shocktheory can well reproduce the bow morphology. We argue that molecular bows arecommon 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 theHI data in the same region to obtain a map of HI optical depth and molecularfraction. We found a firm evidence of HI-to-H$_{2}$ transition in the galacticshock as revealed by a sharp molecular front at the MBS front.

publications of the astronomical society of japan

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