Molecular Bow Shock in the 3-kpc Norma Arm

A molecular bow shock (MBS) at G24.4+00+112 ($l\sim 24\deg.4, b\sim 0\deg, v_{\rm LSR} \sim 112$ km/s) is studied using the 12CO(J=1-0)-line survey obtained with the Nobeyama 45-m telescope at $20''$ (0.71 pc) resolution. The terminal velocity uniquely locates the object at the tangent point of the 3-kpc expanding arm (Norma arm) with the distance of 7.3 kpc. The bow ridge extends over $\sim 160$ pc ($1\deg.3$) perpendicularly to the galactic plane, and is concave to a ring of HII-regions centered on G24.6+00 at the same distance. The edge on the down-stream (higher longitude) side of the MBS is extremely sharp, and is associated with several elephant trunks in gear-to-gear touch with the HII regions. On the up-stream (lower longitude) side of MBS, a broad HI bow is associated at the same velocity. The coherently ordered structure of HI, CO and HII gases indicates HI-to-\htwo transition at the galactic shock followed by efficient star formation due to dual compression, where the molecular gas is shock-compressed from up-stream side by galactic shock and from down-stream side by HII expansion. We propose a scenario of galactic sequential star formation (GSSF) along the spiral arms. We also discuss related ISM phenomena such as the hydraulic jump, bow shock, and Raileigh-Taylor instability occurring around the MBS.